Touch pins and ground (GND)
The capacitive touch pins on your board work as touch sensors by themselves, WITHOUT a connection to GND. You can even attach wires to them to make a bigger sensor with other conductive material.
Touch sensors
Some boards have pins or pads that can work like buttons when you touch them. If you touch one of these pins, you change what’s happening electrically to the pin at that moment. The microprocessor connected to the pin can detect this change and say that is was a button action.
Just like with real buttons, all the same actions are noticed: a press down, lift your finger off, hold your finger down for a long time, and press down multiple times quickly.
Resistive touch
One way of sensing touch is to change the electrical resistance of a material when it is touched. This means that some special stuff is used that allows more or less electricity through it when pressure (like your finger pushing on it) on its surface changes. The microprocessor is connected to this resistive stuff and knows when more or less electrical current flows through it. That’s how it knows you touched it or lifted your finger away. Resistive touch material will work with other things too, not just you finger. A pen, pencil, or something with a pointed tip. If there is a large area of resistive touch material, like a computer display screen with a touch surface, the material can very accurately detect where you touched it anywhere on it’s surface.
Capacitive touch
A very popular way to detect touch is with capacitive touch sensors. Capacitive means using capacitance which is about storing up a really, really small number of electrons of the surface of some material. When you touch the sensor, your body becomes part of the storage surface for the electrons on the sensor. If you have a grounded sensor, the number of electrons, its charge, becomes less when you touch it. If you are using a non-grounded sensor, the charge increases when you touch it. Either way, a sensing circuit can detect a touch by the change in this charge.
In fact, a sensor can be just a pin or small metallic area on a circuit board. Probably just like the pins on your board. The microprocessor has a special circuit to detect a change in the charge at a pin and can know when you touched it. What makes it nice about the capacitive touch pins on your board is that you can just use them like they are buttons.