He meant, roughly, that what we experience can only be representations, not things in themselves, of which we can know nothing except that they must exist in order to ground the representations.
The idealism is 'transcendental' because we are forced into it by considering that our knowledge has necessary limitations and that we could not know things as they are, totally independent of us. But there is nothing to stop us knowing the appearances as they are, presented to us as from outside and not invented by us, for we could have no way of inventing them if they were not really presented to us in this way. (Perhaps compare, though Kant does not say this, the way we cannot invent new colours in imagination.)
I Kant, Critique of Pure Reason, trans, by N K Smith (1953, German originals 1781 and 1787); especially A366-80 (pp. 344-52 in Smith's translation)