In simple terms, chlorophyll makes a leaf look green because it reflects only green light.
Answering in a more detailed fashion, chlorophyll is a pigment. As a pigment it absorbs some wavelengths of visible light, (red, orange, yellow, green, blue, indigo, and violet are the colors of visible light), while reflecting others. In specific, chlorophyll absorbs all other colors of light but green, which it reflects. It is this reflection of green light that cause a leaf to appear green.
