can you explain the theory of the conservation of energy? ie that energy is never lost but simply transfered.?

Answer 1

it means that energy can be converted from one form to another, but it cannot be created or destroyed. In modern physics, all forms of energy exhibit mass and all mass is a form of energy.
The energy conservation law is a mathematical consequence of the shift symmetry of time; energy conservation is implied by the empirical fact that physical laws remain the same over time.

Answer 2

Basically it's motion that is lost by friction transforms into heat.