This is a force-sensitive resistor with a round, 0.5″ diameter, sensing area. This FSR will vary its resistance depending on how much force is being applied to the sensing area.
 
The harder the force, the lower the resistance. When no force is been applied to the FSR its resistance will be larger than 1MΩ. This FSR can sense applied force anywhere in the range of 100g-10kg.
 
Two pins extend from the bottom of the sensor with 0.1″ pitch making it bread friendly. There is a peel-and-stick rubber backing on the other side of the sensing area to mount the FSR. Just Connect a resistor to form a voltage divider and measure the voltage at the junction to find the force applied.
 
These sensors are simple to set up and great for sensing force, but they aren’t incredibly accurate. Use them to sense if it’s being squeezed, but you may not want to use it as a scale.