Some of the details are missing but we can construct our own statement which would explain the situation.
A rock starts on top of the hill and all of its energy is potential energy. When it starts rolling down the hill this potential energy is being used.
Some of the potential energy is transferred to kinetical energy (energy of motion) and some of it is used to work against friction. Force of friction creates heat and this heat is dissipated into the environment so essentially it is lost.
The statement would be something like this:
As rock rolls down the hill, it loses some of its initial energy due to friction.
