What is Energy diagram, turning points?

For a motion along one axis $x$ with conserved $E_m$, the energy diagram plots the curve $E_p(x)$ and the horizontal line $E_m$. Since $E_k = E_m - E_p \geq 0$, the motion is confined to where the curve lies below the line; the gap between them is $E_k$, read directly. At a turning point, where curve meets line, $v = 0$ and the motion reverses.