Unlike animals, plants do not have a "mouth" or a "stomach". Apart from a few plants such as the Venus flytrap, almost all plants do not "eat" things. So how do plants obtain energy and build and repair their own "parts"?
The energy plants need does not come from other organisms, but from sunlight. The chlorophyll in plants can absorb energy from certain wavelengths of sunlight and transfer it to specialized protein complexes, where water in the cells is broken down into hydrogen and oxygen. Although oxygen is extremely valuable to many organisms, it is a "waste" to plants and is released into the air. Hydrogen, however, is valuable to plants and must not be "wasted"; it is used as "fuel" to "generate electricity", that is, to synthesize ATP.
Hydrogen is also used to combine with carbon dioxide to form sugars, the most important of which is glucose. Plants also use these compounds to further synthesize other molecules they need, such as proteins, fats, and vitamins.
Therefore, plants can "manufacture" all the "parts" of their bodies themselves, all they need are the elements to build these parts, and they must be in the form of small molecules. For example, the amino acids that make up proteins all have an "amino group" containing nitrogen atoms. These nitrogen atoms can come from urea or ammonium sulfate. Phosphorus is needed to synthesize genetic material DNA, and phosphate can provide it. The small molecules that supply plants with these elements are fertilizers in the soil.
Therefore, plants only need sunlight, air, water, and fertilizers to grow and reproduce. Since plants can be self-sufficient and do not need energy and "parts" from other organisms, they are called "autotrophs". Animals do not have this "ability" and can only obtain energy and "parts" from other organisms, so they are called "heterotrophs".