Ultra Miniature Coaxial connector what does 50ohm stand for
A 50 ohm connector is designed to be connected with 50 ohm cable, and 50 ohm track on the board. It's a description of the geometry and material of the dielectric used in the cable, connector and track. This controls their impedance, or the ratio of voltage to current of a signal that flows through them. 

You get best results when they are all the same impedance, so that a 50ohm signal launched into the track can continue with the same V/I ratio through the connector and into the cable. 

Small errors in impedance are usually negligible. Large errors can cause loss of signal, and so communication problems.