'Cardiod' is one of the terms used to describe the pick-up pattern of a mic, the others being 'omnidirectional', 'hypercardiod', 'supercardiod', 'shotgun', and 'figure of eight'. This tells you how sensitive the mic is to frequencies around its capsule (the part that transforms sound into electricity). Remember, this is frequency dependant, and for low frequencies, most mics are omnidirectional. A cardiod pattern is a heart shape - and picks up more in front than behind. It will be helpful in a live situation to combat feedback, the downside being that it will 'colour' the sound more than an omnidirectional mic.