Yep it's grooves(lands) cut in the barrel. It gets the bullet spinning so it is more accurate, though from what I understand long ago when they first started rifling it was actually done to help with cleaning and the accuracy gains were a by product.
The twist ratio number you see, 1:9, 1:14 whatever is how many inches it takes the grooves to make one complete rotation. A lower ratio like 1:7 is a fast twist while 1:14 is a slow twist. Bullet weight, caliber, weapon and all kinds of stuff are factors that rifling twist play into.
Bullet weight, caliber, weapon and all kinds of stuff are factors that rifling twist play into.
Actually, it's not bullet weight, but bullet length that affects stability. We tend to think of weight as the factor because until 20 years ago, most bullets were made the same way. With the advent of non-lead monolithic solids, this perception changed. For example, a 30 cal Barnes 165 grain X-bullet will require a different twist to stabilize than, say, a 165 gr Sierra Game King.
As already said the longer the bullet, in a given cal., the faster it has to be spin to stablize it. The old muzzle loaders shooting patch and round ball could be very accurate with a 1 in 72 twist. The new heaver .22 cal. bullets, 70 to 90 gn., must have a 1 in 7 or 1 in 8 twist to be stablized. Another thing that inters into it is the velosity the bullet will be shot at. The faster you push it the longer the twist rate can be. That said, just because they are giong faster does not mean they will be more accurate as barrel harmonics inter into also.