A releaser is a stimulus from one animal to another, which causes a particular response. The releaser, or sign stimulus triggers an innate releasing mechanism in the receiver. The receiver then does its response, the fixed action pattern (FAP).
This kind of system is inherited by both animals. Example: a fledgling bird does its releaser, say it squawks and opens its beak wide, showing bright red inside throat. This triggers the adult to cough up food stored in its gullet. Both 'partners' are behaving instinctually with inherited behaviours: these are not learnt during life. A FAP can truly be said to be 'hard-wired': a specific stimulus nearly always results in the same behavioral response.
Another behavious system is the 'dances' of water birds such as the Great Crested Grebe. They pair for life, and 'dance' whenever a pair meet up after an absence, and on other occasions. The dances are quite complex, and were first described fully by Julian Huxley. Here the meeting is the releaser, and both partners do the fixed action pattern. The function of the system is a little unclear, but it has something to do with reinforcing the pair's bond.
It is not necessary for both partners to benefit, nor to be of the same species. Some moths instantly fold their wings and drop to the ground if they encounter bat ultrasonic signals. This helps helps the moths, but obviously not the bats. In turn, some bats turn of the sound blips when they hear a moth, and glide in the last few feet. This also is an inherited behaviour.
Releaser Facts for Kids. Kiddle Encyclopedia.