An Obsidian Tag is a string which can be associated with an Obsidian Note.

They can be added inline to the content of the note by adding a hashtag symbol (#) followed by the string, or they can be added to the Obsidian Metadata.

It can only contain the following characters:

• Alphabetical letters
• Numbers
• Underscore (_)
• Hyphen (-)
• Forward slash (/) for Nested tags

It must also contain at least one non-numerical character.

Source: help.obsidian.md