This is a resistive touch switch; it uses the resistance of the skin to close a circuit, originally probably enabling a base-emitter current to flow in a (Darlington) BJT that does the actual switching.

You shouldn't read it directly using an IO pin, but use a transistor inbetween.

This is a resistive touch switch; it uses the resistance of the skin to close a circuit, originally probably enabling a base-emitter current to flow in a (Darlington) BJT that does the actual switching.

There are many circuits on the interwebs for this, just google "resistive touch switch circuit".