The word dregs is actually a bit of brewing terminology. The dictionary defines it thusly:
The remnants of a liquid left in a container, together with any sediment or grounds.
After the yeast have completed primary fermentation, they settle down on the bottom, producing a thick mud. The goal of racking to secondary or to the keg is to get your beer off this mud comprised of yeast cells, before it ruins the flavor.  The last bit of beer out of the fermenter presents a bit of a dilemma. You can do one of two things with it:
- Dump it, in which case you lose about a quart of beer.
- Drink it, in which case you’ll find out the next day that yeast is a natural laxative. Yeast are good for you otherwise, though, and have some helpful nutrients.
Tonight I’m opting to drink the dregs. If you really wanted to be gross you could drink it after swishing up the sediment off the bottom, but I doubt most would have the stomach for that. As it is, with a highly floctuating yeast, you can pour a fairly clear glass off of the sediment on the bottom.
The vitamin B in yeast helps prevent hangovers.