As a poet, T. S. Eliot is known for his monumental, if few, works of sprawling and satirical verse. However, lesser known in the mainstream world is Eliot as the literary critic. The theory in your question relates to an essay titled "Tradition and the Individual Talent," in which Eliot details what the incredibly broad term "tradition" means to him and how it relates to English poetry. He then segues into his "impersonal theory of poetry," which revolves heavily around the concept of tradition and the precedence that it takes in the process and labor of a poet.
Eliot defines tradition as a facet of poetry that, while people frequently lament its absence, occurs incredibly rarely in poetry, even in poetry written and praised by those who seem extraordinarily preoccupied with the concept. To Eliot, tradition meant to embody the entire history of English literature in every work. While he understood that poetry was indeed a reflection of the self, he insisted that it was purely secondary to his idea of tradition. This is where the impersonal theory comes in. It is a separation from the self in order to give and devote oneself completely to tradition. Eliot's concept of "tradition" was not simply inherent to English writers; rather, it was an ideal that had to be achieved.