Humidity is expressed at a percentage because it represents the amount of water in the air compared to how much could possibly be held in the air at this temperature. Cooler air can hold less water than warm air. When you reach 100% humidity whether that’s through evaporating more water into the air or lowering the temperature of the air (since then it can then hold less water), the air is said to have reached saturation. This is when there is so much moisture in the air that the air can’t “hold it” so the water starts to condensate on tiny particles suspended in the air.
When a cold front moves in, the relatively cool dry parcel of air A moves towards the warmer moister parcel B. Cold air is denser than warm air so it forces the warm most air from parcel B up and as the parcel’s altitude increases, the amount of water it can hold decreases, but there’s a fixed amount of water in that parcel. When it gets so high in the air and so cold that it can’t hold all the water, it all condensates and creates a cloud. That’s why as a cold front moves, there will be clouds at the front of it.