As you probably know, there are 6 cases in modern Russian. Old Russian, however, had a much more complex system. Many of its features have now passed into obsolescence, but sometimes their relics make their appearance here and there. Some scholars even go as far as to consider these relics to be separate cases, making the total number of cases more than ten. Generally, though, these features are seen as no more than just deviations of the 6 conventional cases, because only a limited number of words has these features. This is one of them.
This feature is a variant of the Accusative and sometimes referred to as the Partitive case. It appears only in a limited number of words, mainly for food and drinks, and conveys the idea of "a part of the substance in question" as opposed to the whole of it. E.g. Желая заварить чаю (=some tea), я набрал воды (=some water) в чайник, но потом передумал и вылил воду (=the water, regular Accusative) обратно в раковину.