Last width is the width of the boot shell at its widest point, usually across the ball of your foot, measured in millimeters. It's the single most important fit dimension for both comfort and control on the slopes.
To find your match, narrow feet typically need 92-97mm, medium feet fit best in 98-100mm, wide feet need 101-104mm, and extra-wide feet require 105mm or more. A boot that's too wide causes your foot to slop around and lose control, while one that's too narrow causes pain and numbness.
A common mistake is buying a boot too wide thinking it will be more comfortable right out of the box. Instead, remember that liners pack out and compress by 5-10% over time. For the best performance, choose a snug fit initially—it will mold to your foot as you ski.