From Team 449 Wiki
A solenoid valve (sometimes just solenoid) is an electromechanical device used to control air flow to pneumatic components. They can usually be classified as single-action or double-action, which refers to the number of "on" states they have.
Single-action solenoids are binary: they are either on, allowing air to flow, or off. This is the simplest kind of solenoid, designed to work with single-action pneumatic cylinders.
Double-action solenoids, in contrast, do not have an "off" state: they shift air from flowing between two different outputs. They are designed for use on double-action cylinders, but can be made to work as a single-action solenoid by routing one output to a loop of tubing.
Solenoids will have diagrams printed on them to show how they work.
3 is the input, 1 and 5 are the exhaust, and 2 and 4 are the outputs.
We read starting from the right box and going left. Each box is a drawing of a state of the solenoid. Input air (3) is directed to 4, and 2 exhausts through 1. 5 is closed.
On the left, in the other state, the ports are in the same places, but are not numbered. This is standard practice. In this state, input air is directed through 2, 4 exhausts through 5, and 3 is closed.