Sometimes, two tags can mean the same thing. For example, pantsu and panties have identical meanings. It makes sense that if you search for one, you should also get the results for the other.
Danbooru tries to fix this issue by using tag aliases. You can alias one or more tags to one reference tag. For example, if we aliased pantsu to panties, then whenever someone searched for pantsu or tagged a post with pantsu, it would be internally replaced with panties. Tags are normalized before they are saved to the database. This means that the pantsu tag only exists in the aliases table.
When a tag is aliased to another tag, that means that the two tags are equivalent. You would not generally alias rectangle to square, for example, because while all squares are rectangles, not all rectangles are squares. To model this sort of relationship, you would need to use "implications"http://chan.sankakucomplex.com/wiki/show?title=help:_tag_implications.
While anyone can suggest an alias, only an administrator can approve it.