A unit is a fixed slice of your bankroll
Your bankroll is the money you have set aside for betting and can afford to lose — never rent, never borrowed money. A "unit" is a fixed fraction of that bankroll, and most disciplined bettors keep one unit at 1% to 2%.
Example: with a $2,000 bankroll and a 1% unit, one unit is $20. Flat betting means most of your bets are one unit. Sizing off a fixed percentage automatically shrinks your bets when you are losing and grows them when you are winning, without any emotional decision in the moment.
Why small units survive variance
Variance is the swing between your long-run edge and any short stretch of results. It is larger and longer than beginners expect. Even a genuinely good 55% bettor will hit losing streaks: the chance of losing five bets in a row at a 55% win rate is 0.45 to the fifth power, about 1.8% — which sounds rare, but across hundreds of bets in a season it is all but certain to happen more than once.
Now compare stake sizes. Ten losing bets at one unit (1%) is a 10% drawdown: painful but survivable. Ten losing bets at 10% of bankroll each can end your season outright, because the losses compound against a shrinking balance. The bettor with the smaller unit lives to place the bets where the edge finally shows up.
Sizing by edge, and where to be careful
Some bettors scale their stake up when they believe the edge is larger — the best-known formula for this is the Kelly criterion, which sizes each bet in proportion to your edge and the odds. We are not going to run the Kelly formula here, because it is unforgiving: it assumes your probability estimates are exactly right, and if you overestimate your edge it tells you to bet dangerously large.
The safe practical rule for most people is flat staking, or a small "fractional" version of edge-based sizing that never risks more than a couple of percent on any single bet. When in doubt, bet smaller. The goal of bankroll management is not to maximize this week — it is to still be betting next year.