Why The type N is a threaded, constant-impedance coaxial connector with a gasket seal, so it keeps water out and maintains a controlled 50-ohm (or 75-ohm) geometry well up into the microwave region, commonly rated useful to about 10 GHz. That combination of weather sealing and low loss at high frequency is why it is the standard choice for outdoor antenna and microwave feedline connections, where a UHF/PL-259 connector would be too lossy and too leaky.
Watch out The small bayonet-style connector described in one choice is the BNC, which twists on with a quarter turn and is typically used up to a few GHz at low power; the PL-259 answer is wrong because type N is a completely different design, not a plated variant of it.
N for Neill, and think 'N = Nice and Number 10': weatherproof, good to 10 GHz.