In Japanese usage, the word mecha can refer to any kind of mechanical device such as a gun or a robot. In the case of Mecha musume they are specifically personifications of military hardware. They should not be confused with other types of moe military mascots, like those representing something abstract like a country's navy or army.
Mecha musume resemble girls wearing armor inspired in its design by some kind of military hardware. Usually the body is exposed with only the limbs encased in armor, though sometimes they wear more and sometimes less "protection".
The design of the armor reflects the weapon system that they are a personification of. Most common are tanks, aircraft and ships, but also anything from ICBMs to military field kitchens. Some carry weapons that also reflect the thing they represent. Mecha musume are usually based on real military systems. Fictitious designs model themselves after historical and modern military equipment.