Tags are for things that cut across folders, like #todo, #idea or #followup. A note lives in one folder but can carry any number of tags.
Adding a tag
Type # anywhere in a note and keep typing. Autocomplete suggests existing tags so you do not end up with three spellings of the same idea. On desktop, accept a suggestion with the keyboard. On mobile, tap the suggested text that appears after the cursor.
Finding tagged notes
On desktop, tags appear in a Tags section of the sidebar, and clicking one filters the note list to notes carrying it. On mobile, the Tags section sits on the Folders screen with a note count per tag. On iPhone and iPad, typing a tag's name in Spotlight finds the tag and the notes that carry it, without opening Grape.
Removing a tag
Delete the tag text from the note. When no notes carry a tag anymore, it disappears from the list.
Keeping tags useful
Tags reward restraint. A handful of tags you actually search for beats fifty categories you invented one afternoon. If a tag would only ever appear in one folder, the folder is already doing that job.