Question

what does 50 percent humidity means

Answer 1

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.

Answer 2

or refer to @thomaster 's explanation here ;D
http://openstudy.com/study#/updates/524b10a3e4b0e0b655986926