As a beer ages yeast settle out to the bottom of the fermentation vessel. This layer of dormant yeast is called "trub." Over extended periods of time the dormant yeast will begin to cannibalize nutrients from neighboring cell walls. This action is called autolysis.
Every two weeks it becomes necessary to separate the wort from the "trub" and this is done by transferring the young beer (or wort) into a new container leaving the dormant yeast layer behind.