Side compression straps are the horizontal straps on the sides of a ski pack that let you cinch down your load. They're especially useful in the backcountry because your pack volume changes throughout the day—you start with layers, food, and water, and end up with less as you shed or consume items. Without compression, a half-empty pack feels floppy and can throw off your balance while skiing downhill.
The main thing to watch for is interference with ski carry attachments. On some packs, the side compression straps sit right where you'd attach skis in an A-frame carry, which can be frustrating. If you regularly carry your skis on your pack, check that the strap placement works with your preferred carry method.
For most skiers, side compression is a solid default—it's the most common system for good reason. Just make sure the straps don't block the features you actually need to use.