What is Roofline model?

The roofline model (Williams, Waterman and Patterson, 2009) bounds the arithmetic rate a kernel of intensity $I$ can attain on a machine of peak rate $P$ and memory bandwidth $B$ by $\min(P,\ I B)$. Plotted on logarithmic axes it is a slanted line (bandwidth) meeting a flat one (peak) at the ridge point $I^* = P/B$.