Full Idea
In his Third Critique Kant established a series of dualisms (pure/practical reason, nature/freedom, epistemology/ethics) but failed to provide a unifying principle; German idealism can be seen as an attempt to provide this principle.
Gist of Idea
German idealism aimed to find a unifying principle for Kant's various dualisms
Source
Simon Critchley (Interview with Baggini and Stangroom [2001], p.187)
Book Reference
Baggini,J/Stangroom,J: 'New British Philosophy' [Routledge 2002], p.187
A Reaction
He cites 'subject', 'spirit', 'art', 'will to power', 'praxis' and 'being' as candidates. This is a helpful overview for someone struggling to get to grips with that tradition.