A signal is any measurable quantity that varies with time (or some other independent variable). If you record the words you speak in a computer memory and plot against time, you get a speech signal. As an example, the above Figure shows a signal for the words "Hello Wireless" recorded in a computer.
Mathematically speaking, a signal is a function of an independent variable. For example, a function can be considered as a signal varying quadratically with time.
Here, we will focus on signals comprising of one independent variable and one dependent variable. The independent variable on x-axis is usually labeled as time, while the value of the signal at each time instant is called amplitude.