A slot is a position in a group, series, sequence or hierarchy. It can also refer to an allocated time and place for an aircraft to take off or land, as authorized by an airport or air-traffic authority:

In computer science, a slot is the operating system’s mechanism for associating an operation with an execution pipeline. This is a key component of very long instruction word (VLIW) computer architecture, where the relationship between operations in a machine and the pipeline to execute them is explicitly defined.

Slot can also refer to an area in a game, such as the space for the goalkeeper in hockey:

Casino managers are frantically trying to maximize their all-important slot revenue without raising their “price” too much. But if players are able to detect these concealed price increases, they will not be inclined to play at the casino, and may even choose to drive across town to another one that offers higher value for their money.

So how do they do it? Thorough testing is essential for the success of a new slot game. It will allow for the identification and correction of bugs, which will ultimately result in a better quality product.