With RBF you can replace an unconfirmed transaction with another identical transaction but with higher fee (the fee will be subtracted from the output) without having to wait for the nodes to drop the conflicting transaction. Non-RBF transactions can't be replaced unless nodes have already dropped the conflicting transaction.

Edit:
Note that not all nodes (and electrum servers) enables RBF. Sometimes your RBF transaction will be rejected even though it is flagged as replaceable. Change servers and try again if that happens.