Use year is the month that your points renew. With an April use year, your 2001 points are good for a stay between April 1, 2001 and March 30, 2002. Your 2002 points are good between April 1, 2002 and March 30, 2003. Your 2003 points are good - well you get the pattern. You can bank current year points to the following use year. If you have an April use year, you can bank up to 100% of your year's allotment of points as long as you do it before the end of September. Up to 50% of your year's allotment of points can be banked by the end of December and up to 25% by the end of January.
Use year is really only important if you think you may need to frequently cancel a reservation, and it has to do with banking deadlines. Let's say that you normally vacation in February and your use year is April. If you have to cancel your February 2002 vacation (which would normally be booked with 2001 use year points), you would have to use your 2001 points before the end of March, 2002 (or lose them) because your banking windows have already passed. Now if you had a September use year and had to cancel that same February vacation, you would still have time to bank those 2001 points if you didn't need them for a trip prior to August 31, 2002. With a September use year, the 100% deadline is the end of February, the 50% deadline is the end of May and the 25 % deadline is the end of June.
IMHO, a use year no more than five months prior to your "usual" vacation time is best. But remember that use year only matters if you have to cancel a reservation and you are past the appropriate banking window.
If this doesn't help, ask another question.