what is a pneumatic signal exactly?
For a pneumatic system, information is carried by the pressure of a gas (or air) in a pipe, instead of electrical current. In antique Greece,  pneuma was used in medicine to describe  air circulating for the  functioning of living systems.

I am not sure the term is very common, although there are several related concepts such as pneumatic actuator, pneumatic controller, pneumatic signal transmitter or amplifier, (fundamentals of) air logic, etc.

By a form of metonymy, I'd consider a pressure signal as the measure, as a given location  over time, in certain units (psi, psig for pound/square inch [gauge]). 

Apparently, due to pressure/temperature conservation laws in constant volume, pneumatic controllers can also work on temperature.

In addition to the above link, the short document Pneumatic Systems describes in Chapter Different kinds of basic circuits: Flow amplification, Signal inversion,  Memory Function,  Delay function.