It's a machine that reads optical discs, the most primitive form of sampling from the '60s.
The shorter wavelength allows it to read discs containing much more information.
From 1978 until 1984, all Laserdisc player models read discs by using a helium-neon laser tube.
To be fair, it can only read Blu-ray discs, it can't burn them.
DVD players also usually read discs at a constant linear rate.
It records and reads discs in all current DVD and CD formats.
The CD drive often did not read burnable or re-writable CD's and other discs with certain formatting.
An 80-track drive will not automatically read 40-track discs.
Some operating systems allow these relaxations as well when reading optical discs.
The 64DD has a 32-bit coprocessor to help it read magneto-optical discs, and to transfer data to the main console.