Multiball is one of the most common features of Pinball. It is when mutiple balls are in play. Multiball usually has two or three balls up at a time. Multiball allows the player to quickly rack up points. It is commonly reached by locking two or three balls.
The ability to have more than one ball on the playing field was actually common place in early pinball, though it was more of a bug stemming from technical limitations. Early machines (roughly pre-1960) could only keep track of the number of balls played by having that many pinballs in the machine. Since balls were loaded and launched manually, the player could have a multiball any time he wanted one.
Once the ability to have the machine count the number of balls played was introduced, multiball generally disappeared for some years. Some machines had it, but it was mostly absent until around 1980 when it became a common feature again. Since then it has become more and more popular, and it is rare in modern times that a machine does not have the ability to achieve it, and most modern machines give players multiple ways to earn it.