Animals may coordinate activities without apparent signal. Air breathing tropical freshwater fish a good example. Low oxygen in ponds, high risk of predation on surface, reduce risk by synchronously breathing. This is a kind of social facilitation. We may not be able to detect the signals that are used.
Meaning is what the recipient makes of the message.
This can only be inferred from changes in the behavior of the recipient. Lets look at one example. Consider a white crown sparrow singing at the top of tree. Only the male sings during the breeding season when testosterone titers are high. Unmated males more likely to sing.