Mixed terrain refers to the typical conditions you'll find at a resort after fresh snow gets tracked out—a combination of groomed runs, chopped-up powder, crusty spots, and soft snow patches. If you ski mostly at resorts and don't chase fresh powder exclusively, this is probably your reality most days.
Skis built for mixed terrain are your classic all-mountain options. They typically feature a medium waist width (around 85–100mm) and tip rocker, which helps them float through soft spots while still holding an edge on firm snow. They're the versatile workhorses of the ski world.
Choose a mixed terrain ski if you spend your days exploring the whole mountain rather than sticking to one type of run. Just keep in mind that while they handle everything reasonably well, they won't match a dedicated groomer ski on hardpack or a wide powder ski in deep snow. Pick based on where you actually ski most, not where you wish you skied.