Tomodachi Life basically follows a bunch of people you create who all live in the same apartment building if and until they get married and move in to one of the neighbouring houses. You watch them do their thing, make friends, go on dates, fall in love, and maybe have some babies.
As you're watching your Mii's do all of this, you can also play some mini-games with them, such as putting on a concert, helping them propose, playing guessing games, or just generally offering them advice when they ask for it. You can change their clothes and their apartments as well.
I adored this game and I played it nonstop for a good while, and I fully intend to go back and play it again. Watching the Mii's grow in to people, engage with others, get married, raise families, the epic rap battles, the cute clothing, the funny item descriptions. All of it was great and so much fun. It can be a major time sink but you really grow to care for these tiny fake humans.