An analogue signal is a signal that varies continuously in both time and amplitude (strength). The variation in time is commonly expressed in the frequency (tone) or frequencies the signal is composed of.
The unit of measurement is the Hertz, or cycles per second. An analogue signal can be e.g. an electric signal that comes out of a microphone.
The opposite of an analogue signal is a digital signal, which is discrete in both time and amplitude. An Analog-Digital Convertor (ADC) is needed to convert an analogue signal to a digital signal.
The term analogue is also used to refer to devices or circuits which handle analogue signals. The output of an analogue device varies as a continuous function of the input.
The first generation mobile communications systems (1G) were analog.