A gate valve has a mechanism that can both allow and block the flow of fluids. It comes with a “gate” that can be wedged into or lifted out of a pipe. When the gate is inserted into the pipe, it stops the flow of whatever fluid is in the pipe. When the gate is lifted, the fluid is free to flow through the pipe.
You can control the gate by taking hold of the handwheel. Using it will allow you to turn on or shut off the valve.