You need to plug an Ethernet cable that is hooked to the Internet into one of the Mesh units. That becomes the master.From there, you just add other units that are able to communicate wirelessly with the master or one of the slaves. When you're all done, you have a mesh network and you can go anywhere in the area that the mesh has coverage without having to worry about which access point you are connected to. Think of these things as a sort of super router that spreads the Wi-Fi signal out. 
You still need an Internet signal for the units to broadcast so you don't save any money on the I-net service.