The middle row of the keyboard is termed "home row" because typists are trained to keep their fingers on these keys and/or return to them after pressing any other key that is not on the home row.
For instance, to type the word poll on a QWERTY keyboard, one would place all of his or her fingers on the home row. (The right hand should be covering "j k l ;" with the thumb on the space bar.) The typist will then use his or her little finger to reach for the "P" key located just above the semicolon and then return the pinky back to the semicolon key from which it originated. The ring finger, located on the "L" key will be moved directly upwards to press the "O" key and then back. Finally, the same ring finger will remain on the "L" key and press it twice. Experienced typists can do this at speeds of over 100 words per minute, but the method is that they always return their fingers to the home row when they are not in use. This provides for quick, easy access to all of the keys on the keyboard.