The term "spawn" can refer to a few different related concepts, all of which involve the (re-)entering of an object into the game world.
Most frequently, players use it when referring to mobs that are no longer present, when they are waiting for them to reappear. (This is often called "respawn.") When an NPC spawns, they typically fade in for a moment, and remain non-aggressive for at least a second or two in order to give the player a chance to react. Some NPCs respawn faster than others.
Spawning can also refer to game objects. Ground spawns are items that are collected directly from the world itself, and appear in specific locations. If a large quantity of these items is needed or if a lot of different players are in the area looking for them, a player might choose to camp a particular spawn point. For example, during the Noblegarden event, many players chose to sit in one place to pick up eggs, and simply waited at that location for them to return.
A few scripted encounters cause enemies or items to suddenly appear in the game that would otherwise not be there. These objects will not respawn, but a player has caused them to spawn once. (Repeating the actions will usually create another batch of those characters or items, but sometimes a period of time must go by first.)