What is Mode 1 (M1) on CD

Mode1 was the first defined block mode that can contain data.

As you know CD was first designed for Audio only, so afterwards ways were thought of to be able to use the same layout but store data in it.

If extracted raw you end up with 2352 bytes per block.  2048 (2K) bytes are user data (the data you actually find in the files).  The remainder of the data contains : sync bytes, header bytes and third layer error correction bytes (EDC and ECC)