Essentially there is no need for a border between any countries in the EU because the EU countries have a Single Market (all goods are the same standard) and Customs Union (import and export duties are all the same).
Trade deals with other countries are agreed by the EU and apply to all the countries in the EU. Trade deals mean you won't apply big tariffs on their stuff and they won't apply big tariffs on yours. So the EU has a free trade deal with Japan so there are no tariffs on Japanese goods coming in and none on EU goods going there. Once any goods come in to any part of the EU it's assumed all tariffs, taxes, customs duties are paid at point of entry so those goods are free to travel around the EU through all the different counties.
So at the moment there's no need for a border between the UK and Ireland so there isn't one. No checkpoints, no border guards, no customs officers. People live on one side and work on the other or sleep on one side and cook on the other. People can completely ignore that there's a border because it doesn't really exist except as a line on maps.
Countries not in the EU are not in the Single Market (the standards of goods, medicines and food etc.) and Customs Union (the collective imports/exports one). Some countries are in one but not the other. They have physical borders with the EU. Norway and Switzerland.
Physical borders are unacceptable in Ireland for many reasons. So the solution proposed was for the UK to remain in the Single Market and Customs Union so that there would be no need for a border to check for crappy sub standard goods or collect customs duties or check for smuggling. But the catch is that if you are a member of the Single Market (free movement of goods) you must also except free movement of people and the UK said they don't want EU people to be able to live and work in the UK. The problem with the Customs Union is that you can't make your own deals with outside countries, you have to stick to the deals the EU makes. Also you have to pay the EU for access to their Single Market and Customs Union and the UK doesn't want to pay the EU for stuff. Especially when as a non member the UK won't get a say in any of the rules and regulations and deals.
So the UK doesn't want to be in the CU or the SM but it doesn't want a border with Ireland either (Ireland doesn't want one either). So the EU said ok, because Northern Ireland is very small and very special for historic reasons we'll allow Northern Ireland to stay in the CU and the SM without paying the money and if you can keep any sub standard goods out of NI and collect any customs duties for goods between mainland UK and NI we'll all be cool. But the Democratic Unionist Party of NI who are propping up the minority Tory government said they don't want there to be any kind of border checks between NI and mainland UK because it's all one country and it wouldn't be right to be treating their part of the UK as different from the rest of the UK. So the EU said ok, you can all stay in the Customs Union without paying until we figure something else out about trading and standards and what not. But while the DUP might be satisfied with this if the whole of the UK was, there are many people who don't want to be in the CU because then the UK can't make its own trade deals. So the DUP are not happy because they fear a future where they are different.
I'm just getting to the backstop so bear with me. As part of the agreement (known as the Withdrawal Agreement) the EU want the UK to agree to Northern Ireland always being special no matter what happens in the future, there won't be a physical border. So in the event the UK decides it wants no alignment with the EU on the standards of food and goods, Northern Ireland will still follow EU standards, and remain in the CU and checks will be done at UK ports. This is only in the event that they are needed. This is the backstop. The DUP don't like this.
Several leading Brexiteers are saying it's perfectly possible to perform all checks and collect all duties somewhere far away from the actual border. So for example, if Thomas James from NI decides to sell chicken to James Thomas from Ireland, he can fill out all the forms saying how much chicken it is and pay his duties and sign the line saying it's chicken raised in NI to EU standards and not cheaper chlorinated chicken imported from elsewhere. And we'll all be fine with that because people never cheat the system and why so many countries with different taxes and regulations don't bother with physical border checks because there's no need. (As it happens there are no countries with no borders anywhere in the world.) These kind of non arrangements are usually called alternative arrangements by Brexiteers. They say there's no need for a backstop because of alternative arrangements and technology which allow border checks to be away from the border.
Added to all of that most Brexiteers don't want to be in the CU at all or have any kind of alignment on food and safety standards, so the backstop would most likely be applied. The DUP are not at all happy about this and the same applies to many other people who feel NI should be treated the exact same way as the rest of the United Kingdom, despite about half of the population of NI seeing themselves as Irish exclusively, and you know 'The Troubles'.
I'm not sure if that's any clearer.