Tags are a flexible way to categorize fonts because they don't set any restrictions on how you organize them. Examples: "Slab serif fonts my wife hates", "Fonts I designed for COMPANY_NAME", "Serif high-contrast text fonts I'm allowed to embed".

Two ways:

The first one is when you import a directory of fonts (and Ask for tag names when importing fonts is enabled in Preferences). A dialog lists existing tags and lets you pick or add a new one.

If you want to assign tags to currently filtered fonts, use Edit → Tag all filtered….

Right-click on a tag in the Tags panel to edit or remove it from the database.