It is reported in feet or metres per hour. It responds to weight on the bit, rotary speed, hydraulics, bit condition, and the rock. A drilling break, a sudden rise, can be a softer formation worth circulating to surface to see. A sudden drop can be a worn bit or a harder streak.
Chasing a higher rate by adding weight without watching torque, vibration, and the equivalent circulating density is how bits and boreholes get damaged. The rate is a measured-depth rate. It is not a vertical progress rate in a deviated well.