Your Use Year is determined at the time you purchase your ownership interest in a DVC resort and marks the time each year when you receive a new allotment of vacation points. It affects when you can bank and borrow vacation points, and sets the deadline for certain vacation decisions.
Use Year only matters if you have to cancel a vacation that was scheduled to occur in the last four months of your Use Year. That's because banking of points is not allowed during the last four months of your Use Year and you don't have much time to reschedule before the points expire. Many owners never have to cancel a vacation, so they don't worry about Use Year.
If you travel about the same time of year on a regular basis, it may be best to choose a Use Year that starts right before that timeframe, or at least falls no more than 7 or 8 months prior to that time. Keep in mind that your travel pattern may change over the course of your DVC ownership. What starts out as a "perfect" use year may end up being not so perfect later on.