Ground is something you should have learned in high school, basic wiring, basically grounding a circuit provides protection to the user/circuit in case of a power spike, it is usually attatched to a metal part of the circuit like in light switches the case is the ground meaning that if there is too much power going to the circuit it'll just discharge over the case instead of blowing up the light bulb.
Or like a lightning rod, they're mounted on top of a house and travel to the ground, so that if lightning strikes the house, it will most likely strike the lightning rod, and travel down the wiring into the ground.
Also, in some schematics, ground can also mean negative, depends on the circuit, if there's no negative in the schematic, it's best to suspect that ground is the negative, it's pretty much guestimation when it comes to other peoples circuits.