If a bonus has wagering requirements, that means it must be played through (or wagered) a certain number of times before you can withdraw any winnings generated from it.
That is why wagering requirements are never a set amount of money. Instead, they will be a multiplier of the bonus you receive. At MadSlots, the wagering requirement on bonus credit is 40x (unless stated otherwise in the promotion).
If your bonus fund balance is £10, and the wagering requirements are 30x, then you have to wager £10 x 30 = £300. Once you have wagered £300 your bonus will be unlocked and converted to real money.
This wagering requirement applies ONLY to bonus credit and winnings from free spins. That means it does not apply to any deposits you make using your own money. Deposits are subject to a separate 1x wagering requirement. This does not apply to Free Spins won whilst playing with your cash credit.