Why Microstrip is a planar transmission line formed right on a printed circuit board: a narrow trace of controlled width running above a continuous ground plane on the other side of the dielectric substrate. The trace width, board thickness and dielectric constant set the characteristic impedance, so designers can print 50 ohm (or other) lines directly into the layout. At microwave frequencies, where connectors and coax jumpers add loss and reflections, these printed constant-impedance interconnects are the practical way to move signals between circuit blocks.
Watch out The idea of tiny coax soldered to the board is tempting because microstrip does the same job, but microstrip has no outer shield at all, just a trace and the ground plane beneath it.
Micro-STRIP = a strip of copper over a ground plane; width plus board thickness set the impedance.