A Range Expander is used to lower the noise in a recording. They lower the noise but don't eliminate the noise. With volume expanders you reduce the signal by a set ratio.
When recording in your home recording studio there will be back ground noise in the recording. A gate will eliminate the noise below a set threshold, an expander is more subtle as it reduces the noise slowly using a set ratio.
You can eliminate the breath sounds of a singer with a gate but it will sound unnatural. With an expander you can reduce the breath sounds, leaving some of the breath sound in making it sound more natural.
Expanders can have the following settings:
Here is a brief explanation of each parameter: