They are the microphone pickup patterns--how the microphone picks up sound. The four patterns are stereo (left and right pickup), cardioid (standard vocal mic pickup), omni (360° pickup) and bidirectional (front and back pickup.)
Depending on what you want to do: if you podcast use cardioid or stereo, unless you have a room full of people use omni. If you're interviewing someone sitting across from you use bidirectional. If you're recording an instrument either stereo or cardioid.