Animals that eat only plants are herbivores. In a food chain, they are called the “primary consumers”. This is because they are the first animals that eat the plants. Energy stored in the plant moves into the herbivore when it eats the plant. This energy is then used by that herbivore.
Think about you eating a fresh tomato out of your family's garden. In that case, you are the primary consumer of the tomato. Your body will use the energy from the tomato. This makes a basic food chain with two links, the tomato and you.
In the wild, a basic food chain would be plants and a bison. The bison is the primary consumer of the plants which are the producers of the energy which comes from photosynthesis which is powered by the sun. Whew! But that is often not the end of the story!