An Internet Protocol Number, or IP, is like a identifcation tool for your PC when it's online.
Weather static (provided by your ISP when you signed up, and unchanging), or dynamic (changing with each reconnection), IP's serve the same purpose in the scheme of the internet. When your computer sends out a packet to the internet, to load a webpage for example, it sends it's IP with it, so the server knows where to send the packet back to.
It also serves as an identication and banning tool on forums, such as this.