The link to the library I used is:
https://www.arduino.cc/reference/en/libraries/capacitivesensor/
This is the library which is used in Project 13 - Touch Sensor Lamp from the starter kit.
A picture of my setup:
There is a piece of metal where I am holding my hand. Things are hold in place with scotch-tape to prevent them from moving.
As far as I understand the library measures the capacitance of the circuit (see the documentation) and this capacitance is affected by any capacitance connected to the circuit. Here connected doesn't have to be an inductive connection but it can also be just a capacitive connection (I hope that makes sense). So the capacitance is different if I run it on a battery or if I connect it to my laptop. You can even see in the serial plotter whether your hands are on the keyboard of the laptop or not.