The term ATP stands for adenosine triphosphate. It has a complex molecular structure - the nucleoside is adenosine, to which is attached a tail containing three phosphates, hence the name. It is considered the energy currency for all cellular activities. Taken on a macro level, it can be considered highly critical to the existence itself of all forms of life on earth, from unicellular micro-organisms to humans. ATP is considered a macromolecule, and second in its importance and relevance to our existence only to DNA.
Being the basic unit of energy, ATP is used for all kinds of activities in the body. The relevance of ATP in our lives is evident if we break down our activities - we perform activities, and require energy for every single activity we perform. ATP is the unit for all this energy. In the human context, it is the most widely distributed compound in our body.
Three different usages of ATP are - building of complex molecules within the body, generating electricity within the nerves that comprise our nervous system, and also expansion and contraction of the muscles in our muscular system.