Bursting means that while you are always guaranteed a certain amount of system resources, far more is available on each box than simply these guarantees. Hence, should you require your loads to spike higher than your guarantees, as long as the resources are available on the box at that time, you can use them, up to your ‘burst’ level.
The burst allotment is not the same thing as your resource guarantee. You cannot count on constant, sustained utilization of your burst resources. If your VPS constantly requires greater resources than your package guarantees, then you must upgrade to the higher VPS or Dedicated server plan.