As you change gears you need to make sure that the chain is as straight (parallel to the bike) as possible to prevent undue wear on it.

If you're on the biggest (outside) front gear you should to avoid the biggest 2 or 3 (inside) gears on the back, because it puts your chain at too great an angle, which is bad because it shortens the chain's lifetime.

Similarly, if you're on the smallest (inside) front gear, you should avoid the 2 or 3 smallest (outside) gears on the back, again to avoid putting the chain at too great an angle.