Sound from a single source arrives at each microphone at a different time. So, a mic that is distant from the source is picking up the source with a delay, which causes variable phase shift vs. frequency. When you combine the close and distant mic signals in your mixer, certain frequencies cancel out due to phase interference, creating a "comb-filter" effect. The frequency response of a comb filter has a series of peaks and dips (see figure below.) This response often gives a thin, hollow, filtered tone quality.

Audible comb filtering can occur whenever two or more mics pick up the same sound source at about the same level but at different distances, and are mixed to the same channel.

This problem can be minimized or eliminated by following 
the 3:1 rule: Separate the mics by at least 3 times the mic-to-source distance. This creates a level difference of at least 9 dB between microphones, which reduces the comb-filter dips to an inaudible 1 dB or less.

In general, place mics close to their sources and keep the mics far apart to prevent audible comb filtering.

The left-side frequency response results when two mics are mixed to the same channel at equal levels, and you followed the 3:1 rule.

The right-side response results when you don't follow the 3:1 rule.