Hey there!
A Budget is a system that helps you to plan how you will spend your money.
For example, say you make about 300 dollars a week. You use 75 dollars on the water bill, 25 dollars on the electric bill, 100 dollars on groceries. You have about 100 dollars left which you put into your savings. You planned this out, and you do it every week. This is your budget (theoretically speaking).
Hope this helps!