Hoon is compiled to Nock, a purely functional turing-complete combinator calculus that can be thought of as Urbit's assembly language or basic opcodes of the virtual machine. Nock is extremely minimalist, with only 11 basic operators, and consequently there are computations that cannot be efficiently implemented in Nock. To allow fast computation in these cases, Nock includes the ability to hand them off to a fast C implementation on the host system. These external implementations are called jets, and any Hoon/Nock code can be jetted in this way.

Further reading