It is the difference between total general government revenue and total general government expenditure defined in the National Accounts system. You can find some sort of explanation at http://ec.europa.eu/eurostat/statistics-explained/index.php/Government_finance_statistics_-_quarterly_data#Government_revenue_and_expenditure
For Greece, Eurostat's underlying figures look like this
UNIT NA_ITEM/TIME 2015Q1 2015Q2 2015Q3 2015Q4 2016Q1 2016Q2 2016Q3
M euro Total GGov revenue 16,945 19,711 19,970 27,484 18,430 20,598 23,780
M euro Total GGov expenditure 20,925 21,336 20,796 34,290 20,222 20,368 20,982
M euro Net lend(+)/borrow(-) -3,980 -1,625 -826 -6,806 -1,792 230 2,798
% GDP Total GGov revenue 42.0 44.5 43.3 61.3 46.2 46.3 50.9
% GDP Total GGov expenditure 51.8 48.1 45.1 76.5 50.7 45.8 44.9
% GDP Net lend(+)/borrow(-) -9.9 -3.7 -1.8 -15.2 -4.5 0.5 6.0
The percentages are of quarterly GDP. If you wanted exact annual percentages then you should add up the four quarterly figures in Euros and divide by the sum of the four quarterly figures for GDP in Euros (from a different Eurostat table), but you will get a close approximation if you take the average of the four quarterly percentages. So for Greece it might represent a government deficit of around 7.7% in 2015
Be aware that the numbers are not seasonally adjusted: it looks as if a lot of Greek government revenue and expenditure comes in Q4.