Organisms, such as humans, take in "substrates" such as glucose, in our food.  During a process called respiration, we react these substrates with oxygen to give carbon dioxide and water; this takes place in the mitochondria of our cells.  Along the way, energy is given out which is used to drive processes which take place within cells (secretion, intracellular transport, muscle contraction etc.)  Originally, energy comes from the Sun.  When producers (a.k.a. plants and certain types of bacteria) photosynthesise, their chlorophyll captures the light energy and they transform this energy into the chemical potential energy of organic molecules such as glucose.  The consumption of plants by animals - consumers in the food chain - allows the energy of glucose to be used for the various functions (secretion, intracellular transport and so on).  This usage of glucose and other substrates occurs through a process called respiration.  The cellular energy is actually stored within a molecule called adenosine triphosphate (ATP).