Ok kiddo, imagine that you have a very important thing to do, but you don't have a lot of time to do it. That's what we call "short time", and it means that you have to hurry up and do things quickly.
For example, imagine you have to clean up your room because your friends are coming to play, but they will arrive in just 10 minutes. That's a very short time, which means you have to put away your toys and make your bed very quickly. You might not have time to do a perfect job, but you have to do your best in the time you have.
Short time is like when you're racing against the clock, trying to get things done before it's too late. It can be stressful, but it can also be exciting to see what you can accomplish in such a short amount of time.