On my root canal bill it says analgesia. So what's that?

Pain Medicine. Analgesia is the term to describe any pain medication given to you so that you don't feel anything during your procedure. Depending on the situation it could refer to local anesthetic, nitrous oxide, any IV sedation, or a handful of other products intended to make you more comfortable. Feel free to ask--i am sure they would be happy to tell you exactly what was done.
Analgesia. Either some pain med or the local anesthetic that was used was line itemed for you procedure.
Shot or medication. This probably is the shot that was given. Sometimes it could be the pain medications that were prescribed.