Switches are the most common sensor around us. The Circuit Playground Express has three switches: two pushbuttons and one slide switch.
There are two large A and B buttons, connected to digital #4 (Left) and #5 (Right) each. These are unconnected when not pressed, and connected to 3.3V when pressed, so they read HIGH. Set the pins #4 and #5 to use an internal pull-down resistor when reading these pins so they will read LOW when not pressed.
There is a single slide switch near the center bottom of the Circuit Playground Express. It is connected to digital #7. The switch is unconnected when slid to the left and connected to ground when slid to the right. Set pin #7 to use an internal pull-up resistor so that the switch will read HIGH when slid to the left and LOW when slid to the right.
This is not an on-off switch, but you can use code to have this switch control how you want your project to behave.
Note that you need to use an internal pull-up for the slide switch, but an internal pull-down for the pushbuttons.
The following pages will show you how to quickly use these switches first in MakeCode then in CircuitPython.