furthest point on the right = 58% efficiency, which is the least garrett include on their compressor maps and considered the maximum they are able to produce.
Read down from the furthest point and you have the max airflow, in this case 47lbs. Garrett estimate 10hp for every lb of air. This is the capability of the COMPRESSOR alone, not taking into account the hot side.
up the right is pressure, obv minus atmospheric pressure to get your boost figure.
In the case of maximum airflow (47lbs), this will be at roughly 24/25psi.
The overlay of rpm in the second graph is to gauge an idea of spool, max boost, and rev limit etc. BUT, doesnt take into account hotside limitations so its not definitive.
The left hand side of the 'island' is the surge line, il guess you know what surge is. This gives an indication of how much boost you can run at a given rpm, when overlaid with displacement.
generally, a nice straight up surge line = good boost response and spool. Notice better spool up than the 3071 its overlaid with.
And thats, pretty much all i know. Theres a chap on vortex who i've referenced who explains these things very well to people who request compressor maps. They arent the be all and end all though, theres obv alot more too it hardware wise that will affect these things.