It is really not too difficult to understand.
After the Brexit referendum Johnson insisted that there would be no need for a border between the Republic of Ireland and Northern Ireland because there would be magical technical solutions available within weeks. The fact that these solutions did not exist anywhere else in the world (there are lots of borders where such a solution would be useful, such as between Turkey and the EU, where trucks can wait for days for clearance) was beyond his comprehension. Johnson thinks that just because he says something is possible, it will magically happen. The head of Brexit planning at HMRC got so fed up with telling him he was talking bollocks (she used slightly less profane language) that she eventually resigned.
In October 2019, Johnson promised the DUP that no Prime Minister would ever agree to there being a border in the Irish Sea - i.e. between Northern Ireland (NI) and the rest of the UK (rUK).
Then a week later he signed the 'oven ready' Withdrawal Agreement (WA) - which placed a border between NI and rUK.
Then he said that any businesses that were told to complete any Customs forms in order to transfer goods from rUK to NI (and in the opposite direction) could just chuck them in the bin.
Then it was pointed out to him that the WA did indeed create the Irish Sea border - so he ignored the issue for a while, before finally coming up with the idea that even though the WA was a legally-binding international agreement, he could break it if he wanted to 'in a very specific and limited way'.
But then Biden won the US election, and being a Democrat and of Irish ancestry, he had a special interest in the RoI/NI/rUK situation (the USA and particularly the Democrats being the negotiators and guarantors of the GFA). Biden told Johnson that if he messed around with any international agreements, he would no longer be seen as a credible partner to any future US-UK trade agreement. Johnson withdrew the offending legislation, attempting to frame it as a goodwill concession the the EU to help negotiations.
Now Johnson has finally signed the Brexit Deal, but he and his ministers still don't seem to know what they've signed. As someone else has pointed out, businesses have consulted their lawyers and HMRC and have been told that they will need to complete customs forms and in some cases, their shipments will be subject to veterinary and SPS checks (which all cost money - £100+ per package has been mentioned). Meanwhile Brandon Lewis (the MP who made the famous 'specific and limited' law-breaking statement) is again insisting that there is no border www.msn.com/en-gb/money/other/parcel-delivery-charges-to-northern-ireland-should-not-rise-after-brexit-because-there-is-no-irish-sea-border-secretary-of-state-insists/ar-BB1clO4o?ocid=msedgdhp
Confused? You should be. Just wait until the reality of what else is hidden in Johnson's Great Brexit Deal begin to come to light.
[For those asking about shipments to other EU countries - yes, the same issue exists. In order to send a £70 commercial package to the EU, the British exporter will need to complete Customs forms, pay a handling fee to the carrier, and pay for certifications - which could add up to another £75 per package. Either the exporter absorbs the cost (which obviously they cannot do as they would make a whopping loss) or the EU customer pays the additional cost (which of course they won't do as it doubles the price of the product). The net result - the British exporter can no longer export to the EU, which is bad news for the owners and the staff if a large part of their customer base is about to be lost at midnight tonight.]