The tone control circuit above works by varying the resistanc between the lowpass and highpass filter end. Imagine the knob is set about half-way, so that the signals are evenly mixed. If you put a 1k resistor across the output you'd reduce the effective value of the upper half of R3 and R2, making the tone brighter. So, the load on the circuit will affect how it sounds.
A buffer is an amplifier with no gain. "What the hell's the point of that?" you ask, annoyed that I'd bring up something so seemingly pointless. Aha - think about it. Ever driven a car with power steering? Ever driven one *without* power steering? The power steering unit doesn't make the steering wheel steer any faster, it just increases the force that you can apply. So, fingertip pressure on the steering wheel can exert half a tonne of force at the road wheel. A buffer amplifier does the same thing - a tiny current can drive a much heavier load, without the circuit generating it being adversely affected by the load.
It also acts as a kind of "one-way valve" that lets a signal pass from input to output but not the other way, so they're used in mixers to prevent the inputs interacting in weird ways.