Bankroll
Your bankroll is the total amount of money you've set aside specifically for gambling. Not your rent money. Not your savings. A separate pool of funds that you can afford to lose entirely without any impact on your daily life. If losing it would cause financial stress, your bankroll is too large.
Bankroll management is about sizing your bets relative to your bankroll so that you can survive natural losing streaks. A common guideline is to never bet more than 1-2% of your total bankroll on any single wager. If your bankroll is $500, keep individual bets at $5 to $10. This gives you enough runway to ride out the inevitable cold patches.
The most important bankroll rule is also the simplest: when it's gone, stop. Don't reload from your savings account. Don't chase your losses with a bigger deposit. Set a session limit and a loss limit before you start, and treat hitting that limit as a hard stop. Coming back next week with a fresh bankroll is always better than desperately chasing tonight's losses.