What is it?
The circulation of a field around a closed curve equals the flux of its curl through any surface bounded by the curve. Green's theorem in 3D, and the integral form of Faraday's and Ampère's laws.
Statement
Where it shows up in computing
A changing magnetic flux through a loop induces a voltage around it — generators and transformers.
Where is it used?
Computing topics reachable from here, through the chain of ideas that leads to them: