A force meter is an instrument to measure the magnitude of a force. This is often just a spring contained within a cylinder that has a scale to mark the extension of the spring produced by the force. The unit of force is the Newton (symbol N) in the International System of Units and in that case the meter may be referred to as a newtonmeter.
A spring-based force meter with uniformly marked scale intervals is relying on Hooke's law which states that the extension (or compression) of the spring is proportional to the applied force.
References
Force meter Wikipedia(Text) CC BY-SA