A silent key is a deceased amateur radio operator; generally silent keys are identified with their call signs in front of their names. The “key” is a reference to the telegraph keys historically used to transmit information, and although the telegraph key is now largely obsolete, its status is so iconic that most people in the amateur radio community understand the reference. Once a silent key is identified, his or her call sign is usually released back into the pool of available call signs.
Amateur radio operators have been around essentially as long as radio itself. They generally broadcast on special frequencies which have been designated for use by the amateur community, and they may use the radio in a variety of ways. Some people hold informal news shows, converse with people on two-way frequencies, or disseminate information which may not be available from other sources. Many hold official licenses and designated call signs.
When an amateur radio operator dies, it may take some time for the rest of the community to become aware that he or she is now a silent key. In some cases, friends or family members of the deceased may communicate this information, and in the era of the Internet, this has been made much easier. In other instances, a prolonged period of radio silence may prompt someone to look into obituaries or news reports which could reveal the radio operator's fate.
Amateur radio operators may also become quite friendly with each other, even if they never meet in person. It is not uncommon to hold a memorial service over the radio, or to reprint an obituary or tribute for a silent key in a radio operators' newsletter. Tributes to silent keys may also include information about their lives beyond the radio, to humanize them for fans who only knew them as voices.
You may hear a silent key referred to as an SK in Morse Code, although “SK” can also signal “end transmission.” Some radio operators actively keep their eyes out for silent keys, so that they can swoop in on coveted call signs when they are made available.
Most amateur radio enthusiast clubs encourage their members to submit reports of silent keys, so that these reports can be reprinted for the benefit of the community. Government agencies which oversee the issuance of licenses may also have forms for the purpose of reporting a silent key so that his or her call sign can be released for use.