The answer is quite simple. Chlorophyll.
Chlorophyll is a pigment that is green in color and found in all green plants as well as other photosynthetic organisms like cyanobacteria. This chlorophyll pigment gives plants their characteristic green color on their leaves and delicate stem tips. Chlorophyll pigments come in a variety of forms and are identified primarily by their chemical makeup, biological activities, and other attributes.
Chlorophyll participates in photosynthesis by transforming light energy into chemical energy. These pigments can be found in the chloroplasts, which are organelles found in plant cells.
Green light is reflected by plants rather than absorbed by them. Thus, plants appear green to our eyes.