Humidity is simply the presence of water vapour in the air.
Relative humidity, on the other hand, is the quantity of moisture present in the air at a certain temperature as a proportion of the maximum quantity of water vapour that the air can hold at that temperature when it is saturated.

At 21°C, 1 kg of dry air can hold up 15.8 gr of moisture and it is said to be at 100% of relative Humidity (rH).

The quantity of moisture that the air can hold changes as the temperature changes, and increases when the air temperature increases. Consequently, while 1 kg of dry air at 21°C can hold up to 15.8g of water vapour, the same quantity of air at -18°C can only hold around 0.92 grams.

If you have 1 kg of dry air at 21°C with 50% rH (that is about 7.9 g), as you cool this air, it will reach saturation (100% rH) at 9.5°C. Conversely, if you take 1 kg of dry air at -18°C and 100% rH (that is about 0.92 g) and raise its temperature to 21°C without adding any more moisture, you will end up with 6% rH ( 0.92/15.8 = 0.06).