To be in jeopardy is to be in danger. Eating three plates of nachos a day may improve your chances of winning your office's nacho-eating contest; unfortunately, it could also put your health in jeopardy.
Jeopardy is a state of being, so this word is almost always preceded by the preposition "in." The phrase "in jeopardy" is just one of several ways to convey that someone is in trouble. You can also try "at risk," "in danger," or — if you're into idioms — "on thin ice," "out on a limb," or "up the creek without a paddle."