When three parts are to be played by two hands, usually the upper two parts are notated in the treble clef and the third part is in the bass clef. (A fourth part, to be played in the pedals, may also appear in the bass clef with the third part).
Since they are notated in the treble clef, the highest two parts are often played by the right hand. While this may be convenient for the sake of reading the score, it is very difficult--and sometimes impossible--to play two independent legato parts in the right hand.
In this situation it is useful to play some of the “inner” (alto) notes with the left hand, if it is able (see yellow highlight). This is called “redistributing the inner part between the hands,” the subject of this lesson.