Optional feature in PCB layout of MMCX board edge connector. What's it for?
That is a rectangular piece of soldermask used as a "solder dam".  It is used in cases where the microstrip ("rf trace") doesn't have a soldermask coating.

During assembly, solder paste is applied to the exposed pads under the connector.  But if the microstrip isn't covered with soldermask then the solder will flow along the microstrip.  This makes for a bad solder joint, and also will change the characteristic impedance of the microstrip.

The solder dam simply acts as a barrier to keep the solder where it belongs.