A track or slider system lets the binding slide along a pre-mounted rail on the ski, so you can adjust for different boot sole lengths without drilling new holes. This is especially useful if multiple people share the same skis, or if you're running a demo or rental setup where boot sizes change constantly.
The trade-off is that track systems add a little weight and can feel slightly less connected than a traditional drilled mount, where the binding is screwed directly into the ski. For most casual resort skiers, that difference is barely noticeable, but aggressive skiers who want the most direct power transfer may prefer a flat mount.
Track systems are also common on "system skis" sold as complete ski-and-binding packages. Just be aware that you'll typically be limited to bindings from the same brand that fit that specific track interface.