Expanders are dynamics processing devices, similar to compressor/limiters, which reduce the gain of an incoming signal by some user specified compression ratio.   
The processing is caused when the level of the signal falls below a user specified threshold. 

The result is an increase in the overall dynamic range of the signal. 

In the case of very large expansion ratios approaching infinity the compressor acts as a noise gate, clamping the input signal level to minimum gain when ever it falls below the threshold. 
See also:
Noise Gate