where does "db" come from?

Almost all the docs do this, I have no idea where "db" came from. I'm also not sure if I should be using web9 or nodejs, but it's often a lot of samples are missing one or both, or are less clearly labeled.

https://github.com/firebase/snippets-web/blob/1c4c6834f310bf53a98b3fa3c2e2191396cacd69/snippets/firestore-next/test-firestore/transaction.js#L8-L23