Okay, what it really means is that an engine’s valves (which allow the combustible air and fuel mixture into and out of the engine) are operated by two camshafts, instead of one as was once traditional.
A camshaft looks like a stick with egg-shaped bumps and those bumps (lobes as they’re correctly known) push against and open the valves at the appropriate time as the camshaft spins.
Okay, so much for Camshaft and Double; what about Overhead?
Well, in the old days, most engines had camshafts located down low inside the cylinder block and opened and closed the valves via a series of levers and pivots. But in a DOHC design, the camshafts are located at the very top of the engine where the lobes can act directly on the valves, saving complexity and weight and allowing the engine to rev faster, which is the way of the world.
By using two camshafts instead of one, it’s easier to make an engine with four valves per cylinder while, at the same time, arranging those valves in a more efficient pattern.
It used to be a big deal, but with a few notable exceptions (mainly old school, large capacity V8s) all engines feature DOHC these days. The popularity of DOHC might be new, but the technology itself is not. As early as 1912, Peugeot had a four-cylinder DOHC design which it used to win the French Grand Prix of that year.
Now find out how suspension bushes work.