Q codes are three letter codes sent using Morse Code (and occasionally, voice) to abbreviate standard questions and answers.  They may be sent as a question, with a question mark, or as an answer to a question; e.g.:

QTH?What is your location?

QTH Sydney – my location is Sydney

A list of common Q codes:

Q codesQuestion/response
QRK1-5The intelligibility of your signals is: 1 Bad, 2 Poor, 3 Fair, 4 Good, 5 Excellent
QRM1-5I am being interfered with: 1 Nil, 2 Slightly, 3 Moderately, 4 Severely, 5 Extremely
QRN1-5I am troubled by static: 1 Nil, 2 Slightly, 3 Moderately, 4 Severely, 5 Extremely
QROIncrease power
QRPDecrease power
QRQ?Shall I send faster?
QRSSend more slowly (…words per minute)
QRTStop sending
QRX?When will you call me again?
QRXI will call you again at … hours (on … kHz or MHz)
QRZWho is calling me?
QSA1-5The strength of your signals (or those of …) is: 1 Scarcely perceptible, 2 Weak, 3 Fairly good, 4 Good, 5 Very Good
QSBYour signals are fading
QSL?Can you acknowledge receipt?
QSLI am acknowledging receipt
QSOI can communicate with … direct (or by relay through …)
QSP?Will you relay to …?
QSPI will relay to …
QSYChange to transmission on another frequency (or on … kHz or MHz)
QTH?What is your location?
QTHMy location is …
QUM?May I resume normal working?
QUMNormal working may be resumed

The full list of Q codes may be found at:

https://en.wikipedia.org/wiki/Q_code