A buffer is a simple amplifier with close to unity gain that adds little or no coloration or distortion to the sound. It basically provides a low impedance output (typically 1 k to 10 k ohms) that can drive a long cable run without your tone getting mucked up by cable capacitance. Treble rolls off according to the RC time constant, the product of resistance times capacitance. You want the product to be low. You can't control the capacitance of your cable except by shortening it, but you have the option of controlling the resistance (impedance) of the output of the last device driving the cable.
Some pedals are "buffered" but with a single transistor circuit, for example the Ibanez TS-10. This does not have the drive capability or transparency of a good quality opamp, hence the TS-10 used with additional stomp boxes can together cause loss of treble when turned off ("tone suck"). This is a common problem for non-boutique pedals (i.e. Boss).
Ideally a pedalboard has the signal running through as few buffers as possible, all of them TRANSPARENT, and all the pedals are bypassed when off. For example, you might have ONE GOOD buffer to drive the signal through all the patch cords, connectors and units on the pedalboard, and ONE GOOD buffer at the end to drive the signal over a long cable from the pedalboard to the amp. This should sound pretty transparent.
If you have pedals with good built-in buffering at the front AND end of the pedal chain, you do not need pedals dedicated to buffering. The only way you can figure this out is to compare your sound with pedals off, and then with your guitar plugged directly into the amp, no pedals at all. Ideally, they would sound the same, with no tone suck from your pedalboard.
On the other hand, if you have a bright rig, a little tone suck might actually improve your sound, so it's all up to you and your ears.
