The phase button in Reaper or on your mic preamp will reverse the polarity of the signal, meaning it will shift it 180 degrees out of phase. So if you're using 2 mics lets say for aggregate drum kit capture (like overheads), it might be fun to pan them hard left and right for a stereo image. Before you do that, still in mono panned up the middle, if you flip the phase on one of the signals you might notice the sound change. It can get hollow or weird sounding (out of phase) or louder and the low end suddenly gains power (closer to being in phase). If you don't hear something like that, the mics are probably close to 90 degrees out of phase, meaning there's probably a lot of "phase issues" (the peaks and valleys of the waveforms don't line up in the same spot causing partial cancellation) that can only be corrected by moving the mics to eliminate the phase issue, nudging the waveform in a DAW to force the peaks and valleys to line up, or by using a phase rotator tool that allows variable phase adjustments instead of 180 degree increments.
It's a very good idea if you have 2 or more mics on a source to flip the phase on one of the tracks just to see what happens. If one position or the other sounds vastly better, doesn't matter which, mission accomplished. If not, it's a good indicator that you have phase issues. Tweaking the mic placement is the preferred way to deal with it. Listen in mono.
There are also a number of processing tricks that use phase. Some of them are solid techniques like mid-side. Others can give your recordings a weird, annoying, hollowed out sound. It's a good idea to train your ears to hear what bad phase relationships sound like. Then become allergic to that sound. You'll make better recordings.
A good example of recording something out of phase would be if you have 2 mics on the top and bottom of a snare drum. It's pretty standard to flip the phase on the bottom snare mic as it's just going to be naturally out of phase with the top.
