Multi-density EPS uses foam molded in different densities—softer in areas that tend to take lighter bumps and firmer in high-impact zones. Standard EPS helmets use a single foam density throughout, which works fine but isn't as finely tuned to different types of hits.
The benefit is more optimized protection: the softer zones cushion smaller impacts more effectively, while the firmer zones handle harder hits. You'll typically find this feature on higher-end helmets from brands like POC.
That said, all certified ski helmets meet the same safety standards regardless of liner type. Multi-density EPS offers an incremental improvement, not a game-changer. If your budget allows, it's a nice upgrade—but prioritize a proper fit and MIPS (rotational impact protection) first, as those make a bigger difference in real-world protection.