All SQL statements open a cursor. An explicit cursor is simply one you've defined and declared before using it. e.g.:
declare
cursor explicit is select * from t;
You manage the opening and closing of it.
Implicit cursors are those where you haven't done this! Oracle manages the opening and closing of it for you. For more details read:
http://docs.oracle.com/database/121/LNPLS/static.htm#LNPLS00602