What is HAR model?

The HAR model (Corsi, 2009) regresses tomorrow’s realised variance on today’s, on the average of the last five days and on the average of the last twenty-two: $\mathrm{RV}_{t+1} = b_0 + b_d\mathrm{RV}_t + b_w\mathrm{RV}_t^{(5)} + b_m\mathrm{RV}_t^{(22)} + e_{t+1}$.