Wobble groove
The guide groove on a blank recordable disc weaves slightly side to side, giving the drive a clock and an address on media that has no data yet.
A blank CD-R/RW or DVD±R has a moulded pregroove that spirals the laser. It isn't smooth — it wobbles at a set frequency, and that wobble is frequency-modulated to carry timing and position. On CD-R the wobble runs at 22.05 kHz and encodes ATIP; on DVD it carries pre-pits or a wobble address. Without it the burner couldn't tell where it was on an empty disc.