The idea is that between around 3pm and 6pm every Saturday, outwith international breaks when international teams play (e.g. World Cup), no TV station in the UK is allowed to air live football. 3pm is the main kick off time.
The idea is that this encourages people to go to their local football team instead of watching the game on TV.
For a small island, there are massive amount of football teams. If you're American you're probably only familiar with the 'Premier League' with teams like Manchester United/City, Arsenal, Chelsea etc. and maybe the Scottish Premier League with Celtic and Rangers, and maybe Championship thanks to 'Welcome to Wrexham' but there are leagues and leagues below, it goes very deep until you get to teams literally just playing on a bit of grass behind a church in a small village and the local gym teacher being the referee.
The blackout is supposed to mean you go to whatever team is nearest or easiest to get to for you, going to big team Premier League games is very expensive if not impossible unless you have silly money to spend or wait years (I mean like 10+ years for a season ticket).
But it doesn't really work for a lot of people. If you moved away for university etc. you might still want to watch your team but the only local team is your team's rival. Or you would end up watching the village football team game.
Most games are aired abroad for expats and foreign fans, but UK residents are unable to watch those legally, hence the growth in fire sticks.
