Every week postmasters/mistresses have to do "the balance", which is like a mini audit/stocktake of the cash in and out of the branch that week, and what remains in the safe (including money-like things such as stamps and electric meter keys etc). Transactions are not just sending the post, but also acting as a portal for government services such as elderly people collecting their pension money in cash every week (as this was before everything was digital and done through online banking/direct debits).
Before the 1990s it was all done with paper records and physically tallied. Around the turn of the century the Post Office (which is a central company controlling things like the brand and the money transport that individual postmasters/mistresses franchise their individual local business from) decided that this was out of date and should be switched to a digital system instead, where transactions were recorded using touchscreens and then the computer would work out the running totals of the transactions itself. Traditionally, if the balance was 'down' (missing money that ought to be there) the franchise holder had to replace it (e.g.: repay the Post Office company) from their own pocket, as it was assumed to have either been stolen or handed out in error when making change etc.
Unfortunately, the Horizon software was faulty, and would reasonably frequently duplicate transactions or only record one part (e.g.: money given out but not what it was for etc). Not only that, but if queried, it would sometimes increase the errors. This led to lots of postmasters/mistresses having to pay the Post Office large amounts of their own money to replace fictional "lost" cash, and/or being accused of theft/fraud, losing their franchise and even being jailed.
The errors tended to go in the Post Office' favour and against the individuals, and it suited the Post Office to assume that everyone was on the fiddle and they were finally bringing them all bang to rights. As a result, there were also 'failures' in the investigation process and the ways in which individuals were treated - and the Post Office interacted with each one as an isolated event, not part of a wider pattern, which might have allowed the victims to realise or prove what was going on sooner, had they known it was systemic.
This has now all come to light and been on the telly, and so lots of people are now very angry about the miscarriages of justice, in particular the people who were wrongly publicly accused of fraud, required to pay money to the Post Office for no reason and had their lives ruined.