INCORRECT chain tension can make for premature sprocket and gearbox wear, rough gear-shifts and snatchy transmission. It can also reduce rear suspension travel and limit the life of the drivechain.
Check your workshop manual for how to adjust your bike's chain to the correct tension. Remember to set the tension with some load on the bike (preferably with someone on it) as the chain will tighten up once a rider's on-board.
Refere to the owner's manual for the correct torque settings for each bolt - that's how much force to use when retightening. You'll need a good torque wrench to do this properly.
Remember to also lubricate the chain while you're at it, as this will prolong its life.
Check this video for a detailed explanation.