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.