When you misinform someone, you give them the wrong information. If you misinform your friend about what homework is due tomorrow, he may do the wrong math problems.
It's possible to misinform people on purpose (deliberately giving them false information), or accidentally. To inform someone is to provide them with information — to educate or advise them about something. Inform comes from a Latin root, informare, "train or instruct," and literally "shape or form." Adding the "wrong" or "not" prefix mis- gives you misinform, "to instruct in the wrong way."