A misguided decision is one that's made without good judgment and careful thought. It would be misguided to think you could write your entire term paper in one night, or to climb into the polar bear exhibit at the zoo for a better look at them.
Misguided is a polite way to say "a really dumb decision or belief." Your choice to go swimming in the ocean during a hurricane would definitely be considered misguided, and so would your decision to join the mountain climbing team if you are afraid of heights. Both are examples of unwise choices made after not enough deliberation. Misguided uses the prefix mis-, "bad or wrong," and guided, or "directed."