Wild is a symbol that is used to replace another one. Think of it as a joker. Ina card game, the joker can be any card you want to be. In slot machines, it acts the same way, but it is automatic. What it means is that the slot machine will determine that best possible combination that the wild can produce. And then it will payout that combination. If you landed only four symbols in a combo, you just need another one to get the highest possible payout for that. If a wild sits on the spot where the fifth symbol should have been, then you are lucky because the slot will think that you landed five symbols on a pay line and not four.