Question

what does 50 percent humidity mean

Answer 1

Humidity is the amount of water vapor in the air.
100 percent humidity is the maximum amount of water vapor the air can hold.
so 50 percent is half of that

Answer 2

To make this easier to explain, I'll define relative humidity.

A measure of how much water vapor is in the air compared to how much it can hold, given as a percentage.

If it is at 50%, then there is 50% of the maximum amount of water vapor in the air that it can hold.