What is the meaning of "CQ"?

The procedural signal "CQ" is used by amateur radio operators to initiate a general call. It effectively means "I am calling any station that can hear me and I am ready to talk." The letters were originally chosen because, when pronounced phonetically, they sound like the phrase "seek you". When you transmit "CQ", you are inviting anyone listening on that frequency to respond to you. This is completely different from indicating that a frequency is in use (which is usually done by asking "QRL?") or conducting an antenna test.