Multisite : what is it?

I have a client who wants to give his clients the option to Log into a specific page of his website, where they can upload and download files.

Is this what they mean by Multisites ?

I found this Plugin for Wordpress but was wondering if WMPU offers something similar so I dont have to Pay for the plugin, which is not a WMPU.
http://wp-client.com

I love this WMPUDEV package and the Great responses I get on SUPPORT but am trying to justify spending $79 a month.