The maximum withdrawal (max cashout) is a limit in the bonus terms on how much of a win arising from the bonus is withdrawable. The rest is deleted at the moment of the withdrawal.
Two forms
A fixed amount — "max withdrawal €100". That is the form which makes a large bonus practically worthless: a €500 bonus with a €100 cap is in reality a €100 offer that takes a €500-bonus worth of clearing to reach.
A multiple — "max 5× the bonus amount". That is fairer, because the cap scales with the offer.
Where it appears most often
On no-deposit bonuses and free spins, almost always. The logic makes sense from the casino's side: when your own money is not at stake, the upper end is capped. See the no-deposit bonus list.
How to factor it into the calculation
An offer's real value is the lower of two: the balance expected after wagering, and the max cashout. If the second is smaller, all the rest of the calculation is theoretical. Use the max cashout calculator and the bonus calculator.
The alternative
Rakeback has no max cashout, because it is not a bonus but a payment — a share of your own stakes back, withdrawable immediately. See the list of houses without wagering.