Servo valves are for controlling the flow of fluid (water, gas, etc.) with electrical signals.
It's easy to get cheap solenoid valves like the ones used on dishwashers but they are typically binary devices - they are "all on" or "all off" and can't do anything in between or turn on and off gradually. They also typically consume quite a bit of electrical power while actuated.
Real servo valves are great, but they are also expensive (hundreds of dollars.)
This thing lets you make a working servo valve from a hobby servo and a plumbing valve! A servo and ball valve might cost $10.
To make use of this thing you will need to find exactly the servo and valve I used (TowerPro MG995 and DAHL 0176, respectively.) You can also adjust the model for your own servo + valve. All dimensions for servo and valve can be changed pretty easily.
Controlling the servo valve with an arduino: https://youtu.be/CpzewsjtqsY