I understand Kant's categories, however I don't understand their value in arguing against skepticism.
I take Kant's transcendental philosophy to be a non-dogmatic (contemporary the contender was the "Spinozian") way out of the Agrippan Trilemma. The Agrippan Scepticism does say that there may be no certainty in any knowledge, because all sceptic questioning leads to one of the three outcomes of the trilemma (it isn't that correct, because Agrippa had five principles, but I will not go into it for now).
Now, as I wrote in an answer here with some textual support, kantian transcendental philosophy asks for the conditions of the possibility of our reference to objects (of representations).
Taking this for granted, scepticism is to some extend (as absolute scepticism and as solipsism) self refuting, because it actually has to use concepts (modern speech: language), sensual intuitions and representations to even formulate its position. By this, space and time as forms of the intuitions and the categories of pure understanding have to be involved.
As you can see, transcendental philosophy of course does not work against all scepticism. It is for itself a highly sceptic philosophy, working against all kinds of metaphysics that proclaims entities without corresponding intuitions, like wolffian metaphysics, which were quite popular back then. But it works against some radical forms of scepticism by virtue of dictating them their conditions of possibility as necessary a priori.
The main value of transcendental philosophy against scepticism can therefore be considered as showing the boundaries of the usefulness of scepticism as philosophical method.