ReplayGain is a method to even out the peak level differences between different audio files so the overall volume levels sound similar between different songs.
It is not an "on the fly" normaliser or Auto Gain Control system. It does not continuously tweak the volume while a track in playing. Instead it applies one specific boost or reduction for the whole track, designed to allow enough headroom so the loudest parts don't so high that equalization or other adjustments might cause clipping distortion.
Also do not expect that volume levels will necessarily be increased; this is not a "goes up to 11" volume knob. The official spec defines the target level as 89 dB sound pressure level, or -14dB lower than digital maximum - although Poweramp does not go quite that far - so you should expect tracks that were originally encoded at, or near to, full volume will become quieter, while very low level tracks will get louder. For a more detailed explanation, see https://en.wikipedia.org/wiki/ReplayGain .
For ReplayGain to work, each audio file must have been previously embedded with special tag data to indicate the maximum levels that will be found within that song. There are lots of easy ways to create and save this data, but for a simple batch process that can do your entire music collection in one go you might like to try foobar2000 on a computer (or there is also an Android app version).
ReplayGain is controlled via the Settings=>Audio>Replay Gain (RG) page, where you can define your chosen target level for any ReplayGain-enabled tracks. You can also set a fixed gain value for tracks that do not contain any ReplayGain data - for example if you know that most of your music is recorded at 100%, you could reduce the volume slightly so those tracks match more closely with your ReplayGain target level.
You can see if the currently playing audio file contains ReplayGain data by using the three-dots menu on the Player Screen (or long-press on the cover artwork) and choosing 'Info/Tags':
Track Gain versus Album Gain
ReplayGain is processed based on one of two possible pre-embedded tags. The Track value is set purely based on that individual track's own maximum peak level. The Album value is worked out to gauge the maximum peak value in any track within that album group. This is done so the applied gain won't vary track-by-track while you are listening to a whole album, and songs that are intended to be played quieter than the rest of the album will stay quieter.
You select which of those two modes you want Poweramp to apply via the Settings=>Audio>Replay Gain page. The Album version is only worth using while you are listening to a whole album. If you are listening to individual tracks from different albums, use the Track option.