can you explain me the concept of 'tension'?

Think it like this way.

suposse a object is being act by gravivitational force and and if  there is no acceleration asociated with the object then there must be some upward force acting the object. Its either normal(contact force ) or the tension force.Tension force is providede only if object is attached to a string.I hope this explains your query.Otherwise post ur qestion clearly what do you want to to know.