Short Answer:
In effect pedal design, true bypass switching means that once an effect pedal is turned off, its output is completely absent from the rest of your signal chain as your signal passes directly from the pedal's input jack to its output jack. Buffered bypass switching means the opposite, where an effect pedal's output is still somewhat present in the signal path, even when the effect is disengaged. There are pros and cons to each type of design to consider.
Long Answer:
Man, one of the age-old questions in effect pedals? It is, in fact, a great question to consider as you go about picking pedals and designing a board. There are a couple things to think about when deciding over true bypass versus buffered bypass switching, so consider the following:
First, some effect pedal fans will swear by the "purity" of true bypass switching designs. What this means is, essentially, with less design circuitry going into a pedal to bypass your signal instead of buffering it, there's better preservation of guitar and amp tone. Buffered bypass pedals specifically feature more involved circuits that are constantly operating, which might mean some interference in your unaffected guitar signal, but also figure that true bypass effect pedals can alter your unaffected guitar signal also.
When it comes to buffered bypass pedals that are constantly operating, taking up more signal "space" with a more involved design, part of that design in buffering is to accommodate for this reality. It is true that buffered bypass effect pedals might be more present in a signal by virtue of their design. Part of that design, however, is a move toward preservation. With buffered bypass pedals, each buffer helps to preserve signal strength in long cable runs and output consistent power while buffering. Some effect pedal purists prefer true bypass switching for its minimized effect on your overall guitar and amp tone, but with the absence of a buffer, like one you'd get in a buffered bypass pedal, longer cable runs can ultimately affect your top-end signal as it is run through true bypass pedal after true bypass pedal. Unfortunately, neither design does it all in terms of completely preserving signal when disengaged.
Ultimately, the choice between true bypass and buffered bypass switching is a matter of taste. True bypass effect pedals have less signal presence when turned off, but buffered bypass effect pedals work to accommodate their increased presence when turned off.