What does a correlogram describe? And, in particular, how should I interpret these two correlograms? Thanks
On the y axis is the autocorrelation. The x axis tells you the lag. So, if x=1 we are looking at the correlation of December with November, November with October, etc. If x=2, we have a lag of 2 and we are looking at the correlation of December with October, November with September, etc.
In the first graph, there are high positive correlations that only slowly decline with increasing lags. This indicates a lot of autocorrelation and you will need to take that into account in your modeling.
In the second graph, the correlations are very low (the y axis goes from +.10 to -.10) and don't seem to have a pattern.
The gray areas are confidence bands (e.g. tell you whether the correlation is statistically significant).