Chlorophyll is what allows plants (and other organisms with it such as algae and cyanobacteria) to do the synthesis part of photosynthesis. When the cell combines the small molecules of carbon dioxide and water into the larger molecules of glucose, the chemical bonds that hold these glucose molecules together require energy. That energy comes from chlorophyll's ability to remove electrons from water molecules and use light energy to boost their energy. (In aquatic plants you can see oxygen bubbles accumulate on the leaves. This oxygen comes from the water molecules that were split.)
The energy in these high-energy electrons is used to make ATP, and as part of hydrogen atoms that get bonded into the glucose molecule. The photosynthesizing cell is able to convert some of the light energy that reaches its surface into the bonds of an energy-rich molecule--glucose. The glucose may get converted into still larger molecules, such as cellulose (in plant cell walls) or starch.
When the plant cell (or a cell from an animal that has eaten it) later breaks the glucose molecule down, some of the energy released when the bonds are broken is used by the cell to perform work, such as moving molecules across membranes or building other, larger molecules. The process where cells release energy form glucose in the presence of oxygen is call cellular respiration.