Reason is negative and dialectical because it resolves determinations of understanding into nothing; it is positive because it generates the universal and comprehends the particular therein. Hegel, Science of Logic, 28/1: 16
What does this mean?
Here's how I see it: reason lacks any prominent characteristics and is subjective, consisting of contradictions. To be honest this is as far as I've got.
I have never really dabbled in philosophy, so if the answer was kept simple, I suppose that would make it easer for me to understand.
The logic of understanding is the logic of non-contradiction : being is, non-being is not; each being is itself, and not other than itself. Understanding is a form of comprehension that is adapted to finite beings, and its concepts are those of traditional ontology, traditional " categories".
But understanding is not even aware of its own contardiction while rejecting the possibility of contradiction. It does not see that the assertion " no being is other than itself", precisely defines each being by a negation, by a reference to the other.
Reason accepts courageously this contradiction, the fact that the negative relation to the other constitutes every being in itself. According to reason, A = A is a result , that is, self identity is mediated by self -alterity : A is not not A, not being A is part of what it means to be A. ( The " aliquid" - the something - is essentially " aliud quid".)
Since every being has its being in the other, every being contradicts itself: the law of being is the law of contradiction, hence the law of mouvement, ofbecoming.
Contradiction is the negative side of Reason. Contradiction overcome ( contradiction contradicted at the level of the Universal, from the point of view of the " Whole") is the positive aspect of Reason.