An Everett branch is one of the "worlds" in the straightforward many-worlds interpretatation of quantum mechanics. On a naïve realist view, the world we perceive around us is all that we have reason to expect exists. However, clever experiments in quantum mechanics tell us that the universe is actually a described by a mathematical object called a wavefunction, with our observable universe existing "side-by-side" with many alternatives, "parallel universes" to put it extremely crudely. It is difficult to explain the necessary concepts in just a few sentences, and true understanding requires a deep mathematical grounding.