Shellac can be made out of different things, and has been a common polish for wooden instruments for example, before we got chemical protections like nitrocellulose lacquer. For handmade instruments [shellac and french polishing](https://www.lmii.com/blog/2017/10/07/finishing-options-for-m...) is still often preferred due to being environment and human friendly (chemical lacks are often non-healthy for the human who works with them), relatively inexpensive (cook your oyster shell :)), and for being do-it-yourself solution which many luthiers prefer. Some people claims a guitar polished with shellac sounds better than one with a cellulose lack, but I am not sure if I agree that is a deciding factor for the sound, probably more of a psychological factor.