Kernel
Shannon is the figure who founded information theory in a single 1948 paper at Bell Labs — "A Mathematical Theory of Communication." Every subsequent digital technology, from compact discs to the internet to wireless to deep learning, rests on his definition of the bit and his quantification of channel capacity. The paper has no antecedents; it has only descendants.
Contribution
The bit as the unit of information. Channel capacity (Shannon-Hartley theorem). The mathematical theory of secrecy systems (1949, the founding paper of modern cryptography). The Boolean-algebra-as-circuit-design move (his 1937 master's thesis, the substrate of all digital logic). Information theory itself.
Lineage
MIT undergraduate, MIT master's thesis, Bell Labs from 1941. The most consequential master's thesis in any field, ever; the 1948 paper is the most consequential single technical paper of the 20th century.
Civilization-scale significance
Without Shannon, no digital civilization. The fact that he is much less famous than physicists of comparable importance says more about who gets to be famous than about what he did.