Tail width is the measurement of your ski at its widest point near the back, measured in millimeters. It doesn't get as much attention as waist width, but it has a real impact on how a ski finishes and releases from turns.
Wider tails give you more support and power at the end of a turn, which is great if you love carving clean, aggressive arcs on groomers. The trade-off is that they can feel hooky—the tail wants to grab and finish the turn rather than letting you slide out of it. Narrower tails release more easily, making them ideal for smearing turns, skiing moguls, or just a more playful, relaxed feel. Freeride skis often feature a pin tail (much narrower than the tip) for that surfy, slashy sensation in soft snow.
Think about how you like to finish your turns. If you want precision and edge hold, go wider. If you prefer a loose, smeary style, go narrower.