What it means
The maximum fee you offered per unit of gas is below the network's current base fee, so no block can include the transaction. Base fee moves with congestion, and a fee that was fine an hour ago can be under the floor now.
How to fix it
Increase the max fee and resubmit, or wait for congestion to fall. Most wallets refresh the suggested fee when you retry, so re-opening the transaction and accepting the new estimate is usually enough.