You can borrow one year's worth of points. You can also bank one year's worth of points. At any given time, you can have 300% of your annual allotment in your account at one time (last year's banked + current year's + next year's borrowed = 300%). You do have to bank by certain deadlines and you can only borrow when making a reservation (makes sense as there's no reason to do so until you have to).