Aussie,
Twist rate is the distance in which it take the lands and grooves within the barrel to make one, full 360 degree rotation. This is important, in that this rate is what determines what bullets a barrel will, or will not, stabilze. The longer the twist, the lighter (shorter) the bullets will need to be to achieve proper stabilization. The longer and heavier a bullet becomes, the faster a twist it will need to remain nose on during flight.
Complex topic, but this is the gist of it. In the case of the 22 rimfires, there's very few twists available, since the 40 grain offering are pretty universal, and they don't get too much heavier or lighter than these. With centerfires, the bullets weight and lengths can go to some wild extremes, thus explaining why we commonly see everything from 1x6" to 1x16" twists in .22 caliber.
Hope that helps!