Globally Unique Identifiers (GUIDs) are 32 hexadecimal digits used to identify assets. They are preferable to file names because the files can be moved without redirectors.
A useful tool for manually generating GUIDs is guidgenerator.com. Note that if the GUID property is omitted from an asset definition file, then the game will automatically assign a random GUID during a successful load.
Legacy ids are 16 bits with a [0, 65535] range, whereas GUIDs are 128 bits with an unimaginably huge range. This allows them to be generated without coordination or registration between developers.