You're sitting down at the dinner table. Your mum serves up dinner. But all she gives you is a bowl of water, a jar of carbon dioxide gas, and a slice of sunlight. If this was your dinner, you'd either be very hungry - or a plant. The other thing you could be is a seaweed.
Seaweeds are plants, too, and use the same things to grow.
They both turn this gas into the gas we breathe in - oxygen. Some things about land plants and seaweeds are the same, but they have lots of differences too.