If you want to do some additional installation e.g. install an extra deb package on the server machine, this is the right place. In this file you can do whatever you want, but you shouldn't do something that will eventually crash the server.
During installation your plugin package will be extracted and afterwards install.sh will be executed. When execution is done, install.sh will be deleted.
Your install.sh may look like:
#!/bin/bash
apt-get update
apt-get install desiredPackage
root
2015-09-06