You can use tags to organise your documents, collections, and saved queries. Tags can be added to documents from the search result page by clicking on the tag icon next to the title of the document. Below the title of each document are a list of the tags associated with it and clicking on them will take you to a search filtered by that tag. Tags can be added to collections and saved queries from their respective tags in your workspace.
You can also filter your searches by tag to help you find previously tagged documents. This option is found under the collections tab on the page filters.
You can always view all of your tags in full on the tags tab within your workspace here: https://lens.org/lens/user/tags