A slot is a dynamic placeholder that waits for content to be placed into it (a passive slot) or which is called by a renderer to fill itself with content (an active slot). A slot can contain a template fragment, a scenario action, or both.
Traditionally, casino gamblers dropped coins into slots. This changed in live casinos when bill validators and credit meters were introduced, and online when casino players began advancing credits with advance deposits instead of depositing cash. Modern electronic machines are a far cry from their electromechanical cousins, with video monitors and 3D graphics. But they still have the same basic structure: a Random Number Generator and an array of stops on multiple reels, each containing symbols. The odds of these symbols appearing on a payline are determined by their weight in the RNG: lower-paying symbols appear more frequently than jackpot symbols, but are less likely to line up.
Unlike table games, which require knowledge of math and probability, slot machines are primarily designed to sell excitement. This is achieved through a combination of hope and variance: a low-volatility machine pays out often but in small amounts, while a high-volatility game is risky but rewards big wins. The house edge a slot machine offers is a function of these dynamics. It is a measure of how much money the slot is expected to return to the player over time, and varies from game to game. It is also influenced by the denomination of the slot (how many lines it has, and how much a single spin costs) and whether it is an online or brick-and-mortar game.