Foam core skis use polyurethane or similar foam instead of wood in the center of the ski. They're lighter and more forgiving, which can actually be a plus if you're just starting out—the easier flex makes turns less demanding and fatigue sets in less quickly.
The trade-off is that foam cores don't have the same pop or energy return as wood, so they feel less responsive and lively on snow. They also tend to break down over time, meaning the ski may lose its shape and performance after several seasons of regular use.
That said, foam core skis are a perfectly reasonable choice for casual, budget-conscious skiers who stick to groomed runs and aren't pushing for high-speed performance. If you're skiing a handful of days a year and want to keep costs down, they'll do the job. Just know that if you progress quickly or want a ski that lasts many seasons, a wood core is the better long-term investment.