Open Tags in the sidebar (Pro) to create labels with a name and colour. Names are letters and spaces only (case-insensitive unique per account).
On a transaction, pick tags from the list (max five). On the ledger, filter by one or more tags; turn on Must match all tags when you need every selected tag on the same row.
Deleting a tag only removes the label from transactions — the money stays. The confirm dialog shows how many active transactions currently use that tag.
When you import a statement, you can also map an optional Tags column so labels arrive with the file — see Importing tags from your file under Importing Transactions.