How does atp supply energy for cellular activities?

ATP releases energy by breaking down the weak phosphate cell present.  The energy released due to the breaking down of the phosphate cell is then utilized. The ATP is converted to ADP. When a cell feels the necessity for energy it releases a phosphate to gather the energy. It is an automatic and ongoing process. 

First of all 
1) The cellular activities are provided by adenosine triphosphate molecules 
2) ATP is one of the adenosine  molecules, out of three phosphate groups.
3) The energy can be released by breaking down the ATP adenosine diphosphate also known as ADP 

Hope this help :)