Why is atp referred to as "the currency of cellular energy"?

In simple terms, ATP is referred to as the currency of the cell because it is what drives the different reactions and functions of cells. ATP or Adenosine Triphosphate has a relatively unstable structure because of the three phosphate groups that are in very close proximity. Image three magnets that are very close together with the same poles facing each other (therefore in a state of repelling).

When one of these phosphate groups break off, a high burst of chemical energy is released. And this is the energy used by cells. The cells cleverly evolved to use the exergonic reaction produced by ATP hydrolysis (adding water to break off the one phosphate group) to power the different endergonic reactions taking place inside the cell.