This is in relation to ceramics – it’s the maker’s mark that you often find on the bottom of a piece of pottery or a piece of dinnerware.
Backstamps are useful to pay attention to because they often tell you the name of the maker, where the piece was made, and maybe even the date or pattern name. Oftentimes you can tell the date of a piece just by what kind of backstamp it has – if you’re familiar enough with the history of the company, of course.
If you’re a seller, these are vital to document in your listings. Also make sure you take a picture of it, so people can see it for themselves!