Sprues create the channels that the hot metal flows through to get to your piece. When you're finished spruing it looks like your piece is on the top of a wax tree. How complex the tree depends on how complex or large the piece. A simple ring will look like it's sitting on top of a wax stem. Then the sprue at the bottom of your tree is sealed to the center of a metal plate that has a hole in the center that's filled with wax. You attach it to that, and then put a open ended metal tube on the plate around the sprued piece. Then it's filled with a plaster like material.
After it hardens, the plate is removed and the tube is put in a kiln where the wax vaporizes at very high heat. That's why it's called"lost wax". Then it's place on the arm of an centrifical caster that has been wound up. A crucible is placed against the hole in the tube that leads to the sprued piece. Metal is put in it and melted with an oxy-acetelene torch. The arm is released, and it spins around very fast. Centrifical force pushes the metal into the hollow mold filling the sprues that it travels through and filling the piece..
Then after it cools, you break up the plaster and you piece complete with sprues is there in metal. (Assuming that nothing in the process screwed up).Then you cut off the sprues and polish it.
It's a hard process to describe. I'm hoping that this is fairly clear.