This is a type of pitch in baseball characterized by its unpredictable flight path. The pitch is also known as a knuckler and involves a special grip.
A common misconception of the knuckleball pitch is that it's thrown with knuckles on the ball. The most common grip involves 'digging' the index/middle/ring finger nails just below the seam of the baseball, leaving your pinkie finger loose, and thumb under the ball. Some pitchers opt not to use their ring finger, leaving it loose with the pinkie.
As a result of the grip, the ball has little to no spin as it moves towards the plate, which contributes to its propensity to change directions midflight. It is important that the pitcher is able to throw hard, velocity is what makes the baseball curve and twist in the air to make the batter miss. Throwing a knuckleball at different speeds causes the ball to react differently, and when mixed in with a regular fastball or other pitches it can really mess with a batter's timing.
This type of pitch can be very difficult for a batter to hit, but can also be equally as difficult for a pitcher to control and a catcher react to the movement.
The first successful pitcher to use the knuckleball in the modern major leagues was Eddie Cicotte, who did so in 1908. There are only two pitchers who currently throw the knuckleball, Tim Wakefield and R.A. Dickey, but due to its control issues, many pitchers tend to stay away.