In Star Trek: The Voyage Home, the mysterious probe is trying to contact humpback whales.
The probe arrives near Earth in the 23rd century and emits powerful signals that disrupt the planet’s atmosphere and energy systems. Spock determines that the signals resemble the songs of humpback whales, a species that became extinct in the future shown in the film.
To save Earth, Admiral Kirk and his crew use the captured Klingon bird-of-prey to travel back to 1986. They locate two humpback whales, George and Gracie, at San Francisco’s Cetacean Institute. The crew must transport the whales into the future while also obtaining enough water for their survival.
A common mix-up is confusing the whales with the film’s time-travel objective in general. The earlier question about which extinct animals Kirk’s crew must retrieve identifies the animals, while this question focuses on the species the probe is attempting to communicate with.