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 hash tag (#) icon next to results listings. Below the result listing of each document are a list of your tags associated with it and clicking on them will take you to a search filtered by that tag.
You can also filter your searches by tag to help you find previously tagged documents. This option is found under the Tags sidebar 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