Explanation of linkage matrix output columns

I've read and reread the help on the linkage function to understand the first 2 columns of the output matrix but it's not very clear. Does anyone have some experience using linkage who can explain what the numbers in the columns mean? The goal I have is to use the tags from the original dataset to understand the dendrogram clusters I'm making and after that, create an average of the values in a cluster. Thanks-