The phrase cogito ergo sum is not used in Descartes's Meditations on First Philosophy but the term "the cogito" is used to refer to an argument from it.
According to many Descartes specialists, including Étienne Gilson, the goal of Descartes in establishing this first truth is to demonstrate the capacity of his criterion - the immediate clarity and distinctiveness of self-evident propositions - to establish true and justified propositions despite having adopted a method of generalized doubt.