What does p stand for in "fp" of with open(filename, "w") as fp:

"fp" stands for "file pointer" and it was a pointer to a FILE structure in C. The name "fp" just sort of stuck.

"fd" was an alternate ad usually indicated an unsigned integer, which was the offset of the referenced file in the "file table" (file descriptor).

1. It's an entirely arbitrary name, but I think you understand that.
2. It's usually file pointer.