The Royal Mail have added barcodes to their regular stamps. After 31 July 2023, regular stamps without a barcode will no longer be valid. You can either use up these stamps before this deadline or swap them for the new barcoded ones.
The stamps that are changing are the stamps that will be very familiar to you. They feature the profile of Her Late Majesty The Queen on a plain coloured background.
Your non-barcoded stamps can be exchanged for the new barcoded version through the Stamp Swap Out scheme.