A bankroll is the amount of money you have set aside for sports betting.
This is important because if you start with an initial bankroll of $1,000 and you lose it all, you should probably take a break. It's a method to abide by so you don't go overboard and dip into your life's savings.
The best way to manage your bankroll is betting a certain amount of money per bet. For example, it's wise to bet one to five units (or one to five percent of your bankroll) on every wager. That's a helpful rule because if you start with $1,000 and bet 10 units or $100 on a team you really like and they lose, your money could go down the drain fast.
Of course, this differs if you plan on gambling on a daily basis or just for the weekend. If you go to the sportsbook with your friends and are okay with throwing away a set amount of money, bankrolls and units don't often matter. However, if your plan is to slowly grow your bankroll, it's best to limit the amount you bet, no matter how confident you are.