Multi-touch capacitive sensing is basically a digitiser with a layer that had a small voltage is applied. When you touch it, electrons flow from you into the display. There is a separate layer that has rows and columns that also have individual voltage sources applied to them, and a capacitor is placed wherever they intersect, and each one is affected by capacitive delta when a finger comes in contact with the electric field.
Your finger reduces the substrate of the capacitor that is created by the end panel with the applied voltages a one plate, the atmosphere as the substrate or the insulator, and your finger as the other plate.