It learns to master level from self-play.
http://www0.cs.ucl.ac.uk/staff/D.Silver/web/Applications_fil...
also his lecture bootstrapping from tree based search
http://www.cse.unsw.edu.au/~cs9414/15s1/lect/1page/TreeStrap...
and Silver's overview on board game learning
http://www0.cs.ucl.ac.uk/staff/D.Silver/web/Teaching_files/g...