To comprehend what is, this is the task of philosophy, because what is, is reason. Source
What does this mean?
One meaning of 'be' (are/am/is/were/was) is 'exist'. In Shakespeare's play, Hamlet asks himself 'To be, or not to be?', or 'To exist (should I continue living), or not to exist (should I kill myself)?'.
So your sentence becomes 'To comprehend what exists, this is the task of philosophy, because what exists, is reason'. Or, to rephrase it 'The task of philosophy is to comprehend what exists, because what exists is reason'. I'm no philosopher, so I don't know what that actually means, but it's very like Descartes' famous saying 'I think, therefore I am'.
IMO “what is” here represents a philosophical issue, that is, checking definition #2 of reason: the power of the mind to think, understand, and form judgements logically, would result that what is is a matter of existentialism, of being and reasoning, sort of ”I think therefore I am”.
In this context what is is not a question, it is a statement: what is is reason that is the capacity of thinking.