What is a txn? How can it be invalid? I read this in a IRC exchange:


  verifies your txns against a random node that you don't control. if for some reason that node went rogue, you might have accepted an invalid txn


What is a txn?
"txn" is short for "transaction".  The abbreviation "tx" is also often used.

For an explanation of what makes a transaction (in)valid, see the links at How to know if a transaction is valid?