AWSUM THX introduces the block to execute on success with PLZ. It is implicitly closed by…
O NOES, which is the exception block. It should be closed with KTHX, but it hits the final KTHXBYE, which (I assert) is a bit of syntactic sugar that closes everything.
OPEN handles non-default-terminal-based I/O. FILE in this case is simply a variable, a file handle for the LOLCATS.TXT file.
INVISIBLE is a print command to the debug console, commonly known as STDERR.