There are really two things people mean when they say Phase: Polarity or Timing Difference.

Below I have screenshots of two kicks (they're from two different sample packs, but these two sample packs literally copied each other's samples so they're exactly the same... shame)



Notice the waveform of the two kicks. The top kick has a peak in the waveform when the bottom kick has a dip. This is a polarity difference. Since these two kicks are identical, they will cancel out when played and there will be no sound at all.

Now if you click your sample and find the 'reverse polarity' button you will notice the polarity is literally flipped. See below, I flipped the polarity of the top kick.



Now let's take a practical example from within your track. Below are two kicks I have layered on a track, before making any adjustments to them (even though they're in the key of F and F# let's ignore that's for now)



Focus on the second half of the kick, notice how the waveforms are opposite of each other? Although you could leave this as is, let's try reversing the polarity.



Again take a look at the second half of the kicks. They are now closer in polarity than before we reversed one of the kicks (note: do not reverse both samples, it will not have any effect).

When I reversed the polarity what happened was the kicks created a cleaner 'thump' sound and overall produced a much higher quality kick combination.

Read more about phase here:
https://www.soundonsound.com/sos/apr08/articles/phasedemystified.htm
http://theproaudiofiles.com/phase/

Yeah, pretty much. Point is, that this example was shown on total 180° phase difference (Which pretty much equals signal inversion. Not quite, but it's considered as equal.) of otherwise identical or similar signals. That's the "perfect" example. Fun begins when those frequencies are not the same, but close to each other. They will cacel each other for a while and strenghten each other for another while. This repeats itself over and over. You'll hear it as a distinct "pulsing".

...btw in mixing it's a bad thing, but it can be also usefull. Original Reese (reece? dunno) basses were based on exactly this pulsing.