In Mahayana, the cognitive defilements prevent 'reality as it is' (inherently empty and dependently arising) to be known fully. In other earlier schools of Buddhism, seeing through the illusion of an independent self was seen as enough. Mahayana takes this a step further and points to the inherent emptiness of any independent phenomena.
In the Yagacara (Consciousness Only) Buddhist school the 'belief in an independent self' is seen as an afflicted defilement and 'the belief in independently real phenomena' as a cognitive defilement. These work in tandem to obscure direct apprehension of reality.
For further reading I recommend Edward Conze - Buddhist Wisdom: The Diamond Sutra and The Heart Sutra
For Edward Conze ...
... Buddhist tradition distinguishes three types of hindrances: karmic, defilement-based and cognitive. Karmic hinderances comprise those residuals of past unwholesome actions that continue to seed the consciousness in unexamined ways. The defilement-based hinderences consist of those fueled by greed, hatred, delusion, and so forth. The third, cognitive hinderances include those that result from belief in things existing separately from the observing consciouness. The Heart of the Universe: Exploring the Heart Sutra
In Western philosophy this Buddhist views comes pretty close to the views expressed by some idealist philosophers who generally argued against a materialist view of reality. Idealism - Wikipedia
Interestingly, this idealist view is being expressed in modern science as well. A prominent proponent of this is Donald Hoffman, a neuroscientist, who proposes reality is actually a network of 'conscious agents', to use his terminology. Nice TED summary by himself here - Do we see reality as it is?