To start using the application, you just need to touch the friendly, round, yellow character (SimSimi) and begin to chat with him. You should notice right away that the things that he tells you are quite logical, and fit in with the conversation you're leading.
You can entertain yourself writing all sorts of silly things and reading his reactions, which will occasionally surprise you and are sure to make you laugh. You can try to make friends with him, flirt with him, threaten him... it doesn't matter, because it's an AI.
SimSimi is one of those funny apps that you can download to pass the time, since its main purpose is mindless entertainment. You will need to be connected to the Internet to use it.