This micro:bit Flex Sensor is specifically designed for micro:bit to detect flexing or bending in one direction. They were popularized by being used in the Nintendo PowerGlove as a gaming interface.
This sensors are easy to use. It will vary its resistance depending on how much it is flexed. The circuit board will then convert the resistance to voltage for easy interfacing to micro:bit. If the sensor is unflexed, the output voltage is around 1.8V. When flexed all the way the voltage reduces to approx. 1V