Think "boat." The weight of the boat increases it's wake. The planes weight and density increased with a full compliment of passenger, baggage and fuel. Wings are heavier with a full load of fuel.
It's all about aerodynamics and displacement. A heavier plane (like a heavier boat) displaces more air than a lighter one.
Ever driven behind a semi truck going down the highway? Ever notice that at some distances while driving behind a semi truck you get more of that "rocking your car" motion than you do at other distances?
Ever notice that while passing a truck on the right or left while going down the highway, when you get right along side it with your car's hood about even with the back wheels of the semi-tractor you get almost ZERO wind noise? Then when you go to pass in front of the truck it gets noisier and you have to correct the car because of the wind?
That's turbulence. One jet flying behind another at some distances get the same effect. There's jet wash, turbulence, and jet wake.
You can experience what jet wash, turbulence and wake feels like driving behind a truck, then going to pass. Drive real close behind the truck, and the "wake" the truck produces breaks the air for you. It's real quiet driving down the road closely following a semi. No air noise due to the truck making the wake (displacing air) for you.
If that jet got caught in the wake of a larger 747 in front of it, there might not have been enough air mass to produce enough lift to keep that plane in the air.
That's what jet wake is all about.
On hot summer days for example, planes have a harder time taking off because the air isn't as DENSE as it is on cooler days.
It's all about air density and lift.