This is a chain wear indicator.

This device can give you an indication of how much your chain has become longer. If the chain links becomes too long, the teeth of the chain rings and sprockets will be worn down by the chain now rubbing metal on metal.

There are numerous commercially available devices like this, usually out of metal.
Usually these device work by inserting an angled slope until it can be inserted no further, or by turning a knob until it cannot be turned any further. But depending on how hard you turn or push, the reading will differ.

My goal was to create a chain tool that always uses the exact same probing force. You simply press the two finger holes together until they make contact.

To compensate for different material, print tolerances etc, use a new chain to test where the needle points to. Remember this as your zero-line. Each line corresponds to about 0.2% of chain elongation.

I replace my chains when they have become this much longer:
8sp ~ 1.0%; 10sp ~ 0.8%; 11sp ~ 0.6%