A customer of mine sent me a Bitcoin transfer, and for the first time ever, I saw a transaction message stating:

This transaction has been flagged as replace-by-fee ( RBF )

What does this mean? The transaction has a fairly high miner's fee ( 169 sat/b ), but this message baffles me. Is there a problem with the transaction itself?

The ID is 9ac722ca8177a5528e945c081d5d9b021f91d4687838e66c82e7b4589173a88e

Please elaborate - is there anything I should be worried about?