While soccer is the most popular sport in the world, American football is arguably the most popular sport in the USA. In the autumn and early winter there are many Americans who spend their Sundays watching up to 3 football games and cheering for their favorite teams (or against one of their team’s rivals!).
While visiting or living in the States, you’ll hear a fairly substantial recap of Sunday’s games at work or school on Mondays. There are also many sports references and expressions used in everyday language; since American football is so popular it’s no surprise that there are terms from the game that are used as metaphors for other situations.