Its a "line level" signal suitable to drive an audio amplifier.
The signal is generated from a 3V3 specially optimized PWM signal (optimized for audio quality), and the maximum amplitude of it is about 1V peak to peak out of a 1000Ohm impedance. as such you should use an input impedance that is (much) higher, or you will generate signal loss. For example if you load it with 1K, then you will use the maximum power out of the port, but the output level will be halved, so don't think in terms of "output power".
Also, If you don't output the maximum signal, the dynamic range of the signal will suffer, so don't use any software controlled volume regulation, but always use the maximum volume setting for best quality, and control the actual audio volume with the audio amplifier.
The point is, the output is NOT suitable to drive any speakers or headsets, or other low impedance devices, as quality and volume will suffer when you try. Only use an amplifier with 10K input impedance, or more.