As mentioned several times, Q codes were originally for communication in Morse code. In Morse code, if the signal has a question mark (dit dit dah dah dit dit) after the code, it is a question. If not, it is a statement. So according to this archived NATO document the meanings are
QNH? What should I set on the sub-scale of my altimeter so that the instrument would indicate my elevation if I were on the ground at your station?
QNH If you set the sub-scale of your altimeter to read… millibars (or hundredths of an inch*), the instrument would indicate your elevation if you were on the ground at my station at…hours.
*Note: When the setting is given in hundredths of an inch the abbreviation “INS” is used to identify the units.
If you want something really obscure, there is the unofficial Q signal insult used among amateur radio operators. QLF? means “are you sending with your left foot?”