Short circuit is when two sources try to drive the same node in a circuit at different voltages or frequencies. In an idealized circuit (such as where the wires are assumed to have zero resistance), current flow from one source to the other is infinite. In a real circuit, current flow rises to a level where "something has to give." This could include heating the wires between the sources or tripping some kind of protection mechanism.

I'm not great with analogies, but imagine a train with two locomotives, and each one is set to run at a different speed. There will probably be a lot of screeching of wheels etc. To make multiple locomotives work on a single train requires engineering them to be synchronized with one another.