Identifier
A String which specifies an identifier used in Minecraft. They are used to refer to items, blocks, status effects, locations in data or resource packs, and a lot of other things.
An identifier consists of a namespace and a path. Namespace and path are separated with a colon. An identifier may only contain lower-case letters (a-z), digits (0-9), -, _ and .. It can include at most a single colon (:) for separation. The path may contain / to indicate "folders", but the namespace may not.
If no namespace is specified, it will default to minecraft.
Read more here: Minecraft Fandom Wiki: Namespaced ID