The point of a tank's armor is to protect the crew inside from bullets, shells, and anything that may be potentially dangerous. The first tanks used metal plates that weren't too thick to protect from bullets, as that was all that was necessary. As the technology advanced, more armor was being put on tanks, so bigger guns were being used to break through this armor. This raised the issue of weight, as you can't just continue to add armor to a tank to make it safe, because the weight would make the tank immobile. The solution, then, had to be far more complicated to allow the tanks to maintain their protection while still being mobile. The solution was sloped armor. While there is no one person accredited for developing sloped armor, it's first uses on tanks were some of the early WWII Soviet tanks. The reason sloped armor is so effective is that it increases the effective thickness of the plate, without adding weight to the tank.
Here is a demonstration of how this is accomplished, and just how effective the technique is:
This plate of armor is only 100mm thick, but because of the way it is angled back, the effective thickness of armor that a shell would have to go through is twice that of the plate. This allows for the armor to be light and effective, making for a durable but maneuverable tank. The Germans hadn't discovered this technique until late into WWII, which is evident in their most iconic tank of the war, the Tiger. All of the armor is flat, and all corners of the tank are right angles. This made it very weak when facing tanks with effectively the same armor thickness, if not more, but were much more maneuverable. The development of sloped armor in the German tanks was very delayed compared to the allied forces, as all of the allies had tanks with sloped armor. This disconnect of technology, albeit in such a simple way, makes sloped armor an interesting technique in the theater of warfare that spanned Europe.
The original Tiger I:
The late war Tiger II tank, featuring armor sloped back at 40°, with a thickness of 150mm:
To calculate the effective thickness of the armor, the equation is as follows:
With T being the effective thickness of the armor, h being the nominal (regular) thickness of the armor, and α representing the angle of impact.