Just what it says. Use this tag for images where you'd like to know the artist (if the artist isn't provided) and hope that someone who knows will come along and tag it.
It might be possible to find the artist with the help of a Reverse Image Search Engine (see Help:Image source).
Do not use this tag if there is a source available, such as the artist's homepage, blog, Pixiv, etc. Read howto:pixiv and create a new artist tag yourself, or ask for help in topic #8199: Artist tagging help.