To rile someone is to annoy or bother them. For example, a friend might rile you by constantly texting when you're trying to have a conversation with her. You can also rile water, meaning you make it muddy by stirring it up.
The verb rile is derived from roil, and they basically mean the same thing. However, rile is more commonly used to mean "annoy" and roil mostly describes stirred liquid. If you rile up your sister, what you say makes gets her worked up, ready to argue with you. When a kid splashes around in the tub, the water is roiled — but if it gets all over the floor, his parents will be riled.