Photosynthesis is the process through which chlorophyll containing plants convert sunlight energy into chemical energy. The process also consumes carbon dioxide and generates oxygen. The process can be chemically written as:

`6CO_2 + 6H_2O + sun light -> C_6H_12O_6 +6O_2`

Glucose is a sugar molecule and more importantly a food source. Plants use this process to generate food that is used by a majority of life forms (that are incapable of generating their own food). Food is the source of energy and is also needed to provide nutrients. Since human beings cannot produce their own food, they are dependent on plants, carrying out photosynthesis, for food. Equally importantly, photosynthesis is the source of oxygen and also removes carbon dioxide from our atmosphere. Oxygen is required for cellular respiration, the process by which food is broken down into energy. Thus, human beings are also solely dependent on photosynthetic plants for their oxygen requirements.

Hope this helps.