This What Is seminar is meant to be a rough introduction to representation theory to be able to follow Geordie Williamson's talk. We will sketch the basic concepts : definition of a representation, Maschke's theorem, Schur's Lemma, character theory and if we have some time perhaps the link between representations and \(k[G]-\)modules. Since Geordie will mention modular representation theory I will give an example in characteristic p, where Maschke's theorem fails.