Your computer communicates with the internet by communicating to your router or modem. For making this communication possible both your router and your modem have an IP address. In case you have multiple devices (e.g. multiple PC's) connected to your router, each has an unique IP address. These IP addresses are used internally at your home network. They should not be confused with your public IP address. Your public IP address is globally unique and used for communication by your router/modem with the rest of the internet.