My husband is a periodontist, I had him type this out for you:
Crown lengthening is a process where the dentist (usually a dental specialist called a periodontist who has taken an additional three years of surgical training after dental school) removes a portion of supporting bone and gum tissue from around the tooth to expose more of that tooth so that the dentist has enough room to place a restoration.
When a tooth breaks or has a cavity that extends past the gum line, the dentist needs tissue removed in order to properly seal the restoration. If the edge of the cavity or fracture is below the gum and the dentist is unable to cover the edge, it is unsealed and will leave a gap. Through this gap saliva and bacteria can reenter the tooth and reinfect it, negating everything you had done on that tooth.
The procedure is typically done with local anesthesia, what most people call novocaine (usually it’s a different anesthetic). The periodontist will use a dental drill (lasers do not properly do the job). You will have stitches afterwards. It is recommended to eat on the opposing side for a week and avoid small crunchy food like nuts and popcorn. You usually will clean the stitches with an antiseptic rinse called Peridex as it would hurt to brush stitches.
I hope this helps