What is a PPP server, and why do I keep getting disconnected from it?

I am using a Huawei e160 USB modem and I regularly lose connection to the internet with the following error: You have been disconnected from the PPP server. It has nothing to do with reception, and I am always able to connect immediately back up. Does anyone have any ideas as to why this may be happening? Or at least explain to me what a PPP server is? Thanks.