Globally Unique Identifiers (GUIDs) are 32 digit numbers used to identify assets. They are preferable to file names because the files can be moved without redirectors.
At the time of writing a useful tool for generating GUIDs is: https://www.guidgenerator.com/
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.