What was done
The authors used temperature data obtained from 616 boreholes on all continents except Antarctica to reconstruct a temperature history of the globe over the past five centuries.

What was learned
If all of the assumptions that go into translating depth profiles of temperature into temporal temperature trends at the surface of the earth are correct, the mean surface air temperature of the globe has risen by approximately 1°C over the past 500 years, with about half of the warming coming in the last century.

What it means
Starting 500 years ago, the earth warmed rather slowly for abut 400 years and then more rapidly for 100 years.  The authors, however, say nothing about why the warming occurred, probably because the data they possess shed no light on this matter.