"Jumping to conclusions" means accepting an explanation too quickly. For example, if a coworker is late for work, you might think that she overslept. However, this would be "jumping to conclusions" because there are other possible explanations for why she's late: she might have gotten in a car accident, for example.
"Jumping to conclusions" is a bad habit. People often warn each other not to do it:
Don't jump to conclusions.