Overview
Dennard scaling (or MOSFET scaling) was the companion to Moore's Law that allowed CPU clock speeds to increase rapidly from the 1970s to the mid-2000s.
The Breakdown
Around 2006, Dennard scaling broke down because leakage current and other physical effects caused chips to generate too much heat as they got smaller. This is why CPU clock speeds have mostly stayed between 3GHz and 5GHz for the last 15 years.
Consequence
The end of Dennard scaling led to the 'Multi-core Era,' where performance is increased by adding more cores rather than just increasing the speed of a single core.