Uni-directional / Cardioid
Uni-directional mics use small ports (read: holes) in the back of the capsule to help reduce the transmission of sound from behind the capsule. The ports allow air pressure from behind the capsule to press against the mic's diaphragm from behind, helping to cancel the same wave's pressure from the front of the capsule. While this helps remove unwanted sounds behind the capsule, it reduces the mic's response fidelity, exhibits the 'proximity effect' where bass is louder as the capsule gets closer to the source, and are more prone to 'pop' when hit with a blast of wind from a plosive 'p', 't', or 'b'.
Omni-directional microphones have no ports in the back of the capsule and will pick up more sound from the room when compared to uni-directional capsules. "Why would I want this?" you say. Omni directional mics generally have a flatter response curve and better represent the true sound of what's being recorded. They are less likely to 'pop' with plosive sounds and do not exhibit the proximity effect (bass gets much louder as the capsule gets closer to the source) like the uni-directional capsules do.
Sound Pressure vs. Distance
As the microphone gets close to the source, the energy it picks up from the sound rises dramatically. This allows the ModMic to drive the signal without much amplification/gain and thus it delivers a loud signal compared to the noise. When a mic is further away, the entire signal+noise must be amplified together.