A budget is a plan for money. It lists where money is expected to come from, that is, income, and where the money will go or expenses.
See the article on budgeting about how to create and keep a budget.
A budget can be used to help make financial decisions. Is there enough money to buy a home or condo? How much can you afford to buy? Keeping track of actual spending against the planned budget lets you get a better understanding of your current finances. Then you can make decisions about where you might want to cut expenses.