What is Delocalisation energy?

Example 16.14 (Benzene)

Six $\pi$ electrons fill $\alpha + 2\beta$ and the pair $\alpha + \beta$: $E_\pi = 6\alpha + 8\beta$. Three isolated double bonds give $6\alpha + 6\beta$: the delocalisation energy is $2\beta$, about twice that of the open chain of six carbons, hexatriene ($0.988\beta$): closing the ring is worth as much again. By symmetry all bond indices are equal ($p = 2/3$), and so are the six bond lengths, $139.7\,\mathrm{pm}$, between those of ethene ($133.9\,\mathrm{pm}$) and ethane ($153.6\,\mathrm{pm}$).