A bankroll is money you have consciously set aside and can afford to lose entirely. Never rent, savings or borrowed money. If losing it changes your life, the amount is wrong.
A unit is a fixed fraction of that bankroll — commonly 1%. Flat staking one unit removes the biggest cause of blow-ups: chasing losses with bigger stakes.
Fractional Kelly (a quarter or a half) is more advanced, and it depends entirely on your probability estimate being honest. Overconfident inputs make Kelly dangerous, which is why full Kelly is almost never appropriate.
Write down a stop-loss before you start: a figure at which you stop for the week, no exceptions. The Crypto Richie bankroll tracker warns you when you cross the limit you set for yourself.