Which symbol you mean? These ones ?
The first one (Rain) is the quantity of rain received during 3 hours.
The second one (Rain accumulation) is the total quantity of rain received during the next 12h, 24h, 3 days... according to the scale at bottom of your screen.
The quantity of rain is expressed in mm (or in inch depending of your settings).
Or this one ?
This is the Air Humidity, i.e. the amount of water vapor in air, expressed in % of the maximum amount the air can hold