From the game's encyclopedia
Armies can be raised as Raiders from Rally Points. Raiders can move into neighboring lands and pillage them for Gold.
In order to Raid, a Raiding Army must stop in a Holding owned by a foreign Ruler and choose to start Raiding — upon doing so they will be stuck Raiding for a some time, after which they will get Gold and pillage the Holding.
Gold will be stored on the Army, and only added to the treasury when they arrive home. If you defeat a Raiding Army carrying Gold you will take it, and they will be unable to Raid you for some time.
Only some Rulers are allowed to Raid, most notably Tribal and Nomadic Rulers.
Specific Raid Intents can be set to obtain different rewards.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Raid is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Armies can be raised as Raiders from Rally Points." It covers Raid Intent. The game also links it as Raiding, Raids, Raider, Raiders and 1 more. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.