GUID is an acronym for Globally Unique Identifier.

Every object in AC has a GUID, your character, NPC's, monsters and items.

You can find the GUID of any item in AC by clicking the item and typing "/ACTGUID" into the chat window. The objects GUID will be displayed in the chat window and copied to the Windows clipboard. You can alt-tab out to ACTool and paste the GUID into the current macro.