Phase is the relationship of two or more signals coming from microphones on the same sound source. Think of it as the timing of the signal. You want the signals to be in time with each other so that they work with each other. If there are differences in phase, it will make the combined signal far more complicated and problems may arise. Make sure that whenever you have more than one track for the same take of the same instrument, you check the phase relationship between the two of them.
One way to do this is with the phase button common on most preamplifiers. The button or switch will be labeled most often with ∅. Flip the switch back and forth while getting your sounds to find the most in phase relationship between the two before you track. Another way to check the phase is to record a short segment with the mics into a DAW, and then zoom in on the waveforms to compare the peaks and troughs. If the sound still seems to be lacking, play around moving the microphones and adjusting their distances from the sound source. If both mics are not the same distance, a good rule is to place the far mic three times the distance from the source as the close mic. This will help eliminate phase issues. Phase can be very destructive to your signals if you are not careful. If you are not familiar in identifying phase problems, you will be tormented by strange sound gremlins that wreak havoc on your signal. Cancellations and comb filtering are common pitfalls when dealing with issues of phase. Be aware of these issues in tracking before they become nightmares in mixdown.