A Gacha is game. A machine is filled with prizes. They have a few common prizes as well as one or two popular prizes. These are called the RARE prizes. The basis of the Gacha is you pay usually 25l or 50l for each pull and you receive a random common prize, however in most games the rare prizes are usually the more sought out prize, and consequently they are harder to come by.
In Japan (and now in the West too) a Gacha is toy machine into which a customer puts money into. They then twist a crank and out pops a plastic egg, inside of which appears a prize, usually a collectible toy figure, which is part of a complete set.