Plants are one of the types of life form that are found on Earth, the scientific study of plants is a branch of Biology called botany. Green plants contain chlorophyll and carry out photosynthesis. Green plants trap light from the sun through photosynthesis and convert solar energy into forms that living organisms can use. Almost all the life on Earth depends on photosynthesis for the energy it needs to survive, herbivores eat plants and carnivores eat herbivores or other carnivores. A small number of ecosystems in dark caves and round hydrothermal vents get energy from chemical reactions that don’t involve sunlight.
When we talk about plants generally we think about land plants with stems and leaves, those are called vascular plants. Plants also include mosses and algae that are too small to see without a microscope. Vascular plants almost all grow on land or in fresh water. By contrast the sea is dominated by different types of algae. Seaweed or kelp is a large type of alga,  Seaweed or kelp is a plant that can be useful as human food and plays a role in the traditional Japanese diet.  Japan has the longest life expectancy in the world and Japanese people who are old today have been eating their traditional diet all their lives.
But the ocean surface also has vast amounts of invisible microscopic algae that provide food for marine ecosystems.