Diacetyl gives beer a slick, buttery flavor, and is an off-flavor that's usually preventable. While diacetyl is part of some beers' style guidelines, it's something that most brewers want to do away with.
In the last 3 or 4 days of fermentation (not after fermentation is complete), increase the temperature of your fermentation chamber by a few degrees. If you're making a lager, you can take it out of the chiller altogether. The rise in temperature will kickstart the yeast back into gear to clean up any remaining diacetyl.