Buckle side tabs use plastic or metal buckles with webbing on each side of the waist to fine-tune your fit. Unlike Velcro tabs—which are common on mid-range pants—buckles won't wear out, collect lint, or lose their grip over multiple seasons of heavy use.
They also give you a more precise, incremental adjustment, which is handy when you're layering up or down throughout the season. You can dial in the fit exactly where you want it, and it stays put all day on the mountain.
The trade-off is that buckle systems tend to show up on higher-priced pants and add a small amount of weight. But if you ride frequently and want a waist adjustment that'll last for years without degrading, buckle side tabs are a solid upgrade over Velcro.