Plants and algae can make their own food by photosynthesis, so they are called producers. They use light energy to build glucose from simple substances, and no other living thing in the food chain does this first step. Algae include the tiny floating organisms in oceans and lakes, as well as seaweeds.
The mass of living material in an organism or a population is its biomass. Producers build up biomass as they grow, because the glucose they make is used to produce other molecules such as starch and cellulose. This biomass is the starting point of a food chain.
Animals cannot photosynthesise, so they get their biomass by eating. Herbivores eat producers, and carnivores eat the herbivores. Every food chain therefore depends on photosynthetic organisms, which is why they are described as the main producers of food and of biomass on Earth.