Wake turbulence is what an aircraft leaves behind it as it passes through the air. Wikipedia has a good introductory article about it, if you're interested. Normally it can't be seen, of course, but when atmospheric conditions are just right the shimmering effect of wake turbulence becomes visible. This video of aircraft landing at Madeira airport gives us a rare glimpse of this phenomenon. I recommend watching it in full-screen mode.
That also illustrates why larger aircraft have zones behind them of 'wake vortex separation', where other aircraft aren't supposed to fly - up to 8 nautical miles in the case of 'superheavy' planes like the Airbus A380. The turbulence they leave in their wake can be so severe as to damage another aircraft or cause its pilots to lose control if it runs into it.