I’ll try and add to @engineeristerminatorisWOLV‘s explanation. Now keep in mind, I got this explanation from ART SCHOOL, so don’t fault me if it’s not totally accurate!
“Ground” or “Earth” as it’s called in the UK, is the path back into the earth (literally), which is an alternate (and shorter) path from the circuit back to the source that created it (the power plant). In theory, when electrons flow through a closed circuit, some electrons that are unspent by the load have “no place” to go, but back to the source (completing the circuit). When they do, they will always find the quickest physical path back to the source.
The ground plug (the 3rd pin) in a house is wired to a common wire ground that actually is wired to a copper spike next to your electrical service; that spike is driven into the earth a couple of feet, and provides that “shorter” path back into the earth, back to the source. As electrons have no speed limit (well, basically the speed of light), what happens with the ground is that the “extra electrons” not used more readily flow into the ground pin, and down to the ground spike, and back to the source of the power, if they are unexpended by the “load.”
When there is a short circuit, the extra electrons don’t flow into the ground, they remain in the system and “charge” the metal surfaces and electrify them. When you touch that surface, you create an additional “load” and your body becomes part of that circuit (because flesh/water is a good conductor of electricity). The exit path out of your body goes down to the earth, which in turn goes through the earth back to the source of the energy (the power plant).
In plugs with only 2 conductors, there is no “extra” dedicated path into the earth, so the free flowing electrons stay in the system of the two wires, and eventually find their way back to the source via those two wires. In the case of 2-to-3 pin adapters, as in the power supplies of computer adapters, the 2-pin plug is sheilded (meaning, there are two wires for hot and neutral, but they are wrapped by a shield that is usually a foil wrapper around both wires). The foil wrapper then gets wired into the ground plug and then you have a complete path to ground.
Ground lift adapters defeat the path to ground, resulting in the two-pin scenario; if any metal in the housing/chassis of the appliance touches the wires, then you have a case where the device’s surfaces can be electrified again, which is why it’s not recommended to break off or defeat the purpose of a 3-pronged plug with those adapters. You’ll notice that the ground lift adapters often have a little hang-tag at the bottom; that metal tag is intended to be wired to the junction box of your outlet, as most J-boxes are connected to the house ground.