Mindtribe have a good write-up on pointers for working with sheet metal for design for your projects. Topics include punching, lasering, and bending, and also features of sheet metal design like louvers, chamfers and fillets, and “deep draw” (new one for me too).
At a very high level, sheet metal punching is similar to using a hole punch on a piece of paper. A top tool traps the material against an appropriately-shaped bottom tool and pushes through, creating a hole (or other feature). In the case of sheet metal, though, your material is up to a quarter inch of steel, and your hole punch is a multimillion dollar, many ton, high precision piece of machinery.