What does this equation mean?

where $\langle$ H$\rangle$ is the mean energy in the evolving state and $E_0$ is the ground-state energy of H [ 2 , 1 ] . Inverting this gives a maximum rate of orthogonal state changes per unit proper time, 2($\langle$ H$\rangle$-$E_0$)/$\pi$$\hbar$ , and Giovannetti, Lloyd, and Maccone showed that for a periodic evolution this rate integrates cleanly: a system with mean energy $\langle$ H$\rangle$-$E_0$ above its ground state can pass through at most $N \leq \frac{2}{\pi\hbar}\left(\langle H\rangle-E_0\right) t$. mutually orthogonal states in elapsed time t [ 3 ] . Lloyd used the same bound to price computation itself: no amount of clever engineering lets a system with energy E execute logical operations faster than about 2E/$\pi$$\hbar$ per second, a ceiling with nothing to do with any particular hardware [ 5 ] .