Krausen (or kraeusen) is a term that refers to two different aspects of brewing beer. The foam that forms on the top of beer that is actively fermenting is referred to as krausen. A fermenter that's at a high rate of fermentation will be covered by a thick layer of grimy looking foam and would be referred to as being at "High Krausen". "Early Krausen" would refer to a fermenter that has yet to be covered completely with foam.
There is also a technique for priming beer (that is, adding fermentable sugar(s) and possibly fresh yeast culture to an aging beer to produce a secondary fermentation in the bottle for the purpose of naturally carbonating the beer) that involves adding fresh wort and yeast to an aged beer that has already undergone primary fermentation. This produces a secondary fermentation that naturally carbonates the beer and that doesn't dilute the hop flavour (as long as hops were added to the krausen brew) This is a German technique, and if the resulting beer is not filtered it is referred to as a Krausenbier.