It was down to the final two games of the season and both teams were on equal points, though Celtic had 4 more goals. So Rangers had to win by 5 to win the league, and hope that Celtic lost or drew... But Celtic are 1-0 up (against Dundee United) and Rangers are 0-2 down (against Aberdeen).
After the Rangers disappointment last week I can only say hahaha!
So Celtic are guaranteed a place in next season's Champions League - excellent.