NPC = Non Player Character - Any character that the players do not control.
PC = Player Character - Any characters controlled by players.
Something important has happened that bears on the adventure, but the player characters were not present when the event occurred, they only learn about it remotely. This can result in many ways, from a non-player character telling them some piece of news, to coming across evidence of this other event.
An existing non-player character makes a surprise action.
A brand new face is involved in the adventure. This may be someone the player characters had expected to meet, or a surprise.
Threads are the goals that player characters are going after. These are the unresolved missions that the players are trying to solve. There could be more than one thread in an adventure, some big and important while others of less consequence.
Just as the previous random event will help player characters solve an open thread, this random event will make it harder.
The random event is so important it actually closes an open thread. To close a thread, the issue is resolved or simply goes away out of the PC's minds.
Something bad, or good, happens to a player character, or non player character.
This is a catchall category for anything that does not directly impact characters or NPCs. The event is not necessarily bad or good. The event can be important, but often it is more atmospheric to the scene.