It's a correlated subquery. It runs for each row in t1
So - take a row in t1, that has a value for year. count all the rows from t2 where year is less than that year, add 1 to it (in this case could be <= without the +). The resultset is each year in t1 plus the above calculation for each of those rows.
========================================== Cursors are useful if you don't know sql. SSIS can be used in a similar way. Beer is not cold and it isn't fizzy.