In video games, "Spawning" is the live creation of a NPC, mob, object or item. [1]
Mechanics
Put simply, when something spawns, it appears into the world and can then be interacted with. NPCs, mobs, objects (e.g crates, chests) and items are all things that spawn.
Timer
The majority of spawns are controlled via timers. These timers are invisible and must be manually tracked by players. Once the timer elapses, the spawn occurs. This is the most common method of spawning.
Spawn Windows
Sometimes, a creature won't have a set timer, but rather a window in which they will spawn. For example, let's say that A Bandit has a standard timer which is 7 minutes. Now, lets say that The Bandit King has a spawn window, which is between 20 minutes and 1 hour. In this case, once the spawn conditions for The Bandit King have been met, after a minimum of 20 minutes and up to one hour after the conditions were, The Bandit King has a chance to spawn.
Generally, when it comes to Spawn Windows, so long as the conditions are met the NPC/mob/object/item will spawn within its given window.
Triggered
Sometimes a spawn can be triggered. The trigger conditions can be any range of things, from killing a specific mob, turning in an item, environmental states like time of day or weather conditions, completing a quest, having a certain item equipped/unequipped or even standing in a certain location.