Each time the laser radiation pass through the active medium, it is amplified, as was explained on “population Inversion” (section 2.6).
Contrary to amplifying the radiation, there are many losses:
A necessary condition for lasing is that the total gain will be a little higher than all the losses.
Loop Gain is
defined as the net gain
(amplification less losses) that the radiation see in a round trip transmission
through the laser. It is measured as the ratio between radiation intensity
at a certain plane (perpendicular to the laser axis), and the radiation
intensity at the same plane after a round trip through the laser.