A technique that most good single barrel bourbon
whiskies use where some of the mash from the previous batch is put into the mash of the next batch. This gives a more consistent whisky
and an "older" whisky, since the yeast is a direct descendant of the yeast used for a batch 100 years ago and traces of older batches will be in the new whisky. (It's still gotta be aged, of course) This is very similar to how sourdough bread
is made, only brewer's yeast
and a selection of mashed up grains that's at least half corn instead of flour.
If you're shopping for bourbon and don't see a brand you recognize, something with a large number of years on the label that says "single barrel sour mash kentucky bourbon whisky" is a good bet, but Tennessee is probably fine, too.