“grand parent” - it’s a way to reference the comment as opposed to user in a thread.

“OP” aka “Original Poster” is usually the first in a thread.

“PSA” aka “Please See Attached” references a link or attachment to the original post, and is used in the title of the first post, i.e, please look at this webpage I’ve linked to and let’s discuss it.

“TFA” aka “The Featured Article” is often used in the discussions and is the same object as the original poster referenced via PSA

Welcome! And thanks to comprev who has laid it all out above.