Section 4.1 explains that only specific frequencies are possible inside the optical cavity of a laser, according to standing wave condition.
From all these possible frequencies, only those that fulfill another condition will be emitted as laser lines:
Only those frequencies (modes) that have amplification above certain minimum, to overcome absorption (as explained in chapter 5), will be emitted out of the laser.
This minimum amplification is defined as lasing threshold.
The condition of minimum amplification means that the amplification is equal to losses, so that in a round trip path inside the cavity GL = 1.