Glucose is a source of food energy used by most living plants and animals on the planet. Chemically it is a member of the sugar family with a chemical formula of C6H12O6. A sugar is defined as a chemical composed of carbon, hydrogen, and oxygen, generally in the ratios of 1:2:1 of C:H:O, so glucose is a six carbon member of the sugar family.
In terms of where it originates, green plants produce their own glucose through a process known as photosynthesis. With this process, plants can take the carbon from carbon dioxide (CO2), hydrogen from water (H2O), and oxygen from both to produce glucose and molecular oxygen (O2). They use energy from sunlight to accomplish this. Animals cannot produce glucose in this way so they must obtain it by eating plants. Glucose is a chemical that biological systems can easily use to synthesize ATP, which is used to store chemical energy until needed for later.