What does it mean when the brain has cognitive limits?

Cognition is the process of knowing or understanding.

All brains have cognitive limits; even Einstein's. This is because brains are finite and it is not possible for anyone to understand or know everything there is to know in this universe.

Some people understand or know more than others, depending on things like intelligence, neurological status, age, or how something is explained. For instance, you can't expect a baby of 10 months to understand quantum physics. This is because of age related cognitive limits.