When a zero turns up, the player has two options: Reclaim half the bet and lose the other half. Leave the bet (en prison = in prison) for the next spin of the roulette wheel for an all or nothing gamble. If the subsequent spin is again zero, or does not match the imprisoned bet, then the whole bet is lost.
Zero is neither red nor black, high (passe) nor low (manque) in roulette, nor is it counted as either even or odd. Mathematically, zero is even, but if roulette treated it as such, then pair would be marginally an odds-on bet paying off at even money, which would be a loss to the house.