Title says most, a raid is a map with multiple bosses that you progress through in order to get the loots! You can get creative in this topic with pics etc. if you want/are able to draw things(which I am not).
Here's an example:
Bridge of Doom raid - Vanilla (Stock AI), Lightweight-Middleweight raid. A dark style raid with lots of fire and evil bots :P 3 different paths to get to the end boss (Devil II obviously who drops the feared red extender).
Wiki link to arena's:
https://gametechmods.com/RA2wiki/index.php?title=Category:Arenas
