You can't really read this one line in isolation and get the meaning. The King says:
Now is the winter of our discontent
Made glorious summer by this son of York;
And all the clouds that low'r'd upon our house
In the deep bosom of the ocean buried.
The winter of our discontent was indeed a bad time, but the King is saying that this bad winter has been made into glorious summer by his brother becoming King.
For a good discussion of this opening of Richard III, see the enotes link below.