This term can be confusing, since up until this chapter we have used the word ball to refer to the physical baseball. We've tried our best to use the word baseball instead of ball to describe the equipment. In the context of this baseball concept, balls refers to a pitch that is outside the strike zone that the batter does not hit at. Balls can only be called if the batter does not swing, since they are determined by the pitch's location relative to the strike zone, and the strike zone is automatically disregarded if the batter hits.
Walks (Base On Balls)
If the pitcher throws four (4) balls during a batter's at-bat, the batter is automatically entitled to first base. This occurrence is called a walk or a base-on-balls. If a baserunner is already on first base when a batter draws a walk, that runner advances to second base.
If strikes are like a punishment to the batter for not hitting a reasonable pitch, balls are like a punishment to the pitcher for not having good aim and throwing a pitch that the batter cannot reasonably hit.
To understand this better, imagine if the concept of balls did not exist. Pitchers would throw pitches way above the batter's head or out of reach of the batter's bat, and no one would hit or score any runs. Balls keep the pitcher in check, preventing him from throwing too many unreasonable pitches, and allowing the batter a fair chance to hit the pitch.