Have you jumped the gun before? Has anyone said that to you before? When you have jumped the gun, it means that you have done something early and prematurely.
A: Cindy looks unhappy this morning.
B: She must not finished her assignments last night.
A; Don't jump the gun to make that conclusion. She might just be tired.
In this example, A is tell B not to make the conclusion too early without understanding entire situation.