A betting strategy built on martingale doubles the stake after every loss, so that one win recovers everything lost plus the first stake. On paper the plan cannot fail; in practice a run of six or seven losses, which comes sooner than people think, asks for a stake sixty or a hundred times the first one. Table limits or the bankroll end the sequence long before the promised win arrives.
Bankroll management units turn that share into a number. One unit is usually one or two percent of the bankroll, and every stake is counted in units: a normal pick gets one, a strong opinion perhaps two, and never ten. Counting in units makes results comparable across weeks and bet sizes, and it shows at a glance when stakes have started to drift upward after a winning or losing run.
A betting strategy with Kelly staking sizes each bet by the edge the bettor believes they hold: the bigger the gap between their own probability and the price, the bigger the stake. The formula is exact, but it trusts the bettor's estimate completely, and an estimate that is slightly too kind means stakes that are too large. That is why most who use Kelly bet a fraction of it, half or a quarter.