Many circuits (linear power supplies being one exception) involve the processing of various signals, either analog or digital. For testing (or operation of some) of these circuits a signal source is needed. This can be provided by a signal generator of which there are several types:
RF generators provide high frequency sine waves.
Audio generators generate low frequency, low distortion sine waves.
Pulse generators provide square (digital) or pulse signals.
Function generators provide various types of signals, depending upon their design, and typically includes sine, square/pulse, and triangular waveforms.