I've heard the term transaction 'pinning' several times, but I'm not sure exactly what it refers to.
Transaction pinning happens when:
If the child transaction in (3) is large (eg a commercial service sweeping up lots of transaction outputs), then the total fee that I'd need to pay for a valid RBF would be very large.
In this scenario, my original transaction has been 'pinned' by the child transaction.
Russell O'Connor has proposed changing the RBF policy rules to alleviate this problem.