My bankroll is assets dedicated to gambling. In theory it's what if you lost, you would never gamble again. In reality, mine if probably set at a level that if I lost it all, I probably would not gamble for 3-5 years. I would not be happy.
If gambling is your only income, if you bankroll goes away, you find a job.
If you have a job, you can add to your bankroll. I tend to do that. I take a small portion (say 10-20%) of investable income and add it each year.
If you don't set aside a certain amount of money to gambling, you have no idea how much to place on a wager.