To be imprudent means lacking self-restraint when it would be wise to have it, like that time you started dancing on the table during a math test. Remember that? Everybody else does.
Prudent means "wise or shrewd," but add the prefix im- meaning "not," suddenly you have an adjective that describes the opposite of being wise or shrewd. An imprudent person doesn't think about the consequences and might cross the street without looking both ways or use a curling iron while taking a bath. Careless, wild, imprudent behavior can get you into big trouble!