What unit of measure does a psychrometer measure? it is RH but what does that stand for?

The psychrometer is a device used to measure the

Relative Humidity (RH) of the air.

Relative humidity is the ratio of

(the amount of moisture in the air, here and now)

to

(the amount of moisture the air could hold at this temperature).

Being a ratio, it has no units. It's just a naked number, always

less than ' 1 ', and usually expressed as a percent.