What it means to say that energy is conserved as a ball fall towards ground

Mechanical energy = Potential Energy + Kinetic Energy

if there is no friction, M.E. is conserved
as a ball falls, it is losing height. (P.E.=mgh , therefore P.E. will decrease)

the potential energy is transformed into kinetic energy which equals 1/2MV^2.

the increase in K.E. equals the decrease in P.E.

and the sum of both equals M.E.

thus mechanical energy is conserved.

hope this helps you.