What we see as humidity in weather forecasts is more specifically relative humidity (RH).

It ranges from 0% to 100%. 0% humidity means there’s practically no water vapor in the air. 100% humidity implies that the air is saturated with water, meaning it has the highest concentration of water vapor that can be held at that specific temperature. Any addition of water vapor will lead to it condensing into a liquid creating a cloud or fog. This can also occur when the water concentration is held constant but the air temperature drops. Colder air can hold less water valor which results in the excess concentration of water condensing again into fog. That’s how most clouds form where humid warm air from the surface rises and it expands due to lower pressure, causing it to cool (through adiabatic cooling) and the now excess moisture crashing out of the gaseous phase.