You supply the url_title and it returns the id(s) it has been assigned, as well as the ability to separate the id's by using the separator parameter

Can be used within or outside of channel entries tag

Use as follows:

example: {exp:title_cat url_title="{url_title}" separator="&"}

url_title (required), can manually put in the url_title or use within channel entries tag, and doing something like url_title="{url_title}" separator (optional), breaks up the ids (if there is more than one returned), so if you did separator="&", it'd return 1&2&3