Aside from being a scatological innuendo, Pou is a virtual pet/Tamagotchi game/activity. If you have ever wanted to get a pet cat or a micro-pig, you can safely use Pou. Thus you can acquaint yourself with a light version of what it means to take care of a living being. Actually it's even simpler than that.
Pou only has 4 stats to worry about. Food, Health, Fun and Energy. These stats will decrease in time and can be replenished by doing different activities. For food and health, you will need to feed Pou and administer medicine. These items can be bought with coins. Fun can be sustained by playing games and feeding candy (bad example this last one). There are dozens of games which vary from puzzles to dexterity based. There are many clones of well known game like Bejeweled and Flappy Bird. However, many contain the time aspect, in which the time limit can be extended by playing good or hastened by doing the wrong stuff. There's even Sudoku in there, but it isn't timed, fortunately. Energy issues are solved by turning off the lamp in the Bedroom.
Taking care of Pou, will help it grow and unlock different customization options. Once it reaches certain levels, you might want to check out the Closet to change Pou's appearance and attire. If you can't wait to try out everything then you can buy an adult potion and make Pou a big...pou. You can also use a potion to shrink it back to baby size, although I don't get why you would do that, especially since these potions cost real money.
I must say that besides the virtual pet part, I have enjoyed several of the games found in the Game Room. However, my general dissatisfaction with Pou's name and general appearance was strengthened when I saw this:
I find this design rather intentional.
Pou is okay if you are over 18 and still have too much time on your hands. Playing with Pou may get addictive. Maybe that's why it's so popular. But honestly I would never try it for real. I would stress parents to pay attention to the scatologic and sexual content of this app. I may have only discovered one of each, but there may be more naughty bits hidden around.