Twist rate is the number of rotations, almost always ( 1 ), in a given number of inches. In this case, 1 in 9 means the bullet will complete one full rotation within the barrel for every 9 inches of barrel travelled. If you have an 18 inch barrel, the bullet will make 2 full rotations before leaving the bore.
As to twist rate being good or bad, that is almost wholly in relation to the grain bullet you are using. I am no expert, of that I can assure you, so you need to ask the reloading guys, but generally, certain twist rates are better suited for a certain range of bullets.
This has a good discussion of twist rates that I think will help considering your intended purchase.
Twist refers to the rifling in the barrel and how many complete 360 degree rotations the bullet makes in a given length. A 1:9 twist means the bullet rotates once completely every 9 inches of barrel travel. This is a very typical twist ratio for ARs and it is excellent for most uses.
The rate of spin relative to the weight of the bullet determines stability and thus accuracy. If you're going to fire between 45 and 60gr ammo, 1:9 is good.