Cap construction means the ski's top sheet wraps down over the edges, forming a cap shape rather than having vertical sidewalls. This build method is lighter and less expensive to manufacture, which is why it's common on entry-level and budget skis.
For beginners, cap construction is perfectly fine. You're still developing edge control and likely not charging hardpack at high speeds where edge hold really matters. The lighter weight can actually be a plus when you're learning to maneuver and turn.
The trade-off is that cap skis don't grip as well on firm snow and aren't as durable long-term compared to sidewall construction. Once you progress to carving icy groomers or skiing more aggressively, you may start to feel the difference. But for your first seasons on the slopes, cap construction keeps costs down without holding you back.