voltage gain. How much amplification is applied to the source signal is amplified, if the volume knob is at max.
The volume knob attenuates the source signal before the amplification stage. The gain stage (amplification) is fixed. So if the incoming voltage is a snapshot in time (like a camera) and it is 0.1V coming in. A 2x gain will result in 0.2V output and a 5x gain will result in 0.5V output.
Depending on what headphones you have or will get, you may want 2x or 5x. 5x is generally the default, if used with higher impedence headphones like Sennheiser HD580, HD600, HD650, Beyer DT880, DT990, etc. If all your headphones are 32-ohms with 100+ sensitivity you may want to consider 2x.