A "budget" is a limit on the amount of money that someone can spend.
Your personal budget is based on how much money you make, and how much you need to save each year.
Companies and organizations have lots of different budgets for different things. For example, a company might have a technology budget for computer equipment, a travel budget for business travel, and so on.
Here are some ways to talk about a budget:
He needs to learn to live within his budget.
We have a ten thousand dollar per year budget for education and training.
I heard that our budget is getting cut.