"Tan" may refer to the following:

• The color tan, a light brown. Tan on its own to refer to the color is not allowed, but tags such as tan_fur or tan_background to refer to an attribute with this color for a character or a post is; it's likely that someone forgot the underscore to tag one of those things.
• tanned, for characters who have gotten their skin or body darkened.
• tanning, for the action of getting tanned.
• tan_(artist), an artist using this name on Fur Affinity and Twitter (the latter under the handle @blacksea227).