A particle with the total energy E is moving in a potential energy region U(x). Motion of the particle is restricted to the region when

total energy TE =KE+PE

consider a body dropped from a height h, 

at the dropped point , TE=PE

at the point where the body reaches the surface, TE=KE, here PE =0

hence $$\begin{gathered} PE \le TE \\ \Rightarrow \\ U\left(x\right)\le E \end{gathered}$$