Renormalization is a collection of techniques used to study a system at different length scales and it can seem quite complicated. It is very important in both quantum field theory and condensed matter, for example in the study of the standard model. In this talk I will focus on renormalization as it is used in perturbative quantum field theory, where it is used to get rid of inconvenient infinities, and explain how it is performed by using a simple mathematical example (no complicated QFT equations) and the very basics of Feynman diagrams.