Attributes are basically a way to pass information about a player's or NPC's appearance to other players. There are 30 total attributes, numbered 1-30. Generally, player.attr[1] is reserved for hat images, and on most servers player.attr[3] is reserved for carried objects like bushes, but beyond that it depends on the server. If you set a player.attr to a gani, it will display that gani at the player's position and follow them around. You can configure ganis to show sprites based on attributes as well, which is how the carry gani works. It looks at attr[3] in order to get the image name of the carried object.

You can also send text and numbers through attributes. For example, the fishing rod on Ol' West uses attributes to pass the lure's position to other players, allowing them to see each other's lures move around in the world.