Mean motion is how many orbits a satellite completes per day — the inverse way of stating its period. A TLE's mean motion of 15.5 rev/day means a ~93-minute orbit. Because period follows from orbit size, mean motion also encodes altitude: bigger numbers mean lower orbits.
Understanding Mean Motion
From revolutions to altitude
Kepler's third law makes the conversion mechanical: period squared scales with semi-major axis cubed. A mean motion of 15.5 rev/day is a 92.9-minute period, which solves to a ~6,800 km semi-major axis — about 420 km altitude. This is how tracking software turns the plain numbers of a TLE into the geometry it propagates, and why two satellites with matching mean motion necessarily share the same orbit size regardless of shape or tilt.
The drag terms beside it
A TLE pairs mean motion with its first time-derivative (and a rarely used second): the rate at which revolutions per day are increasing. For a stable high orbit the derivative sits near zero; for a satellite in the thickening air below 400 km it grows steadily, and re-entry prediction is substantially the art of extrapolating that acceleration through an atmosphere that swells and shrinks with solar activity.