It's an international calling and distress channel. Anytime there's a boat on the water that wants to call another boat on the water, they call on channel 16. If there's a distress call, that happens on channel 16 as well, but everybody else has to keep quiet. If they don't keep quiet, the Coast Guard radio, MCTS, will tell them to keep quiet. It will be seelonce mayday, which is an adaptation of the French expression. It's the message that everybody who has taken their restrictive operating certificate will understand to mean, “Keep quiet. Mayday in progress.”
On April 12th, 2016. See this statement in context.