These days it's a round plastic handle with a carved rubber bottom that pairs with a spongy red-ink filled bottom. There's a picture and an explanation at [1]. It basically functions the same way as the signature of an authorized representative of the company does in the West.
Presumably it comes out of personal seals which are a fun item to get if you're in China. I never interacted with someone who used them, but it's cool. See pictures at [2] and history at [3].
"Chop" is also frequently used instead of "seal". I assume its not onomatopoedic, but that is a fair description of the "thunk" sound produced to when the seal is quickly pounded against the document on the table.
[1] https://www.china-briefing.com/news/company-chops-in-china/
[2] https://www.chinahighlights.com/travelguide/culture/chinese-...