Cognitions are content given by the intuition and thought by the understanding, and so goes the motto: "thoughts without content are empty, intuitions without concepts are blind." However, we have concepts that cannot determine any intuition, and so cannot be cognized. These concepts can still be thought meaningfully, despite the inability of their object being given in any experience.
When no intuition can be provided for a concept it means that the object thought by this concept is not determinable in space and time. Some prominent examples of these concepts in metaphysics are: soul, world, God. Other concepts that cannot be represented are: justice, wisdom, forgiveness, &c.
(We can represent any of these intelligible objects via symbols which can be intuited, but these are never fully adequate to the concept.)
The parent of the distinction is concepts of objects considered generally. Concepts of objects generally can be divided into intuitable objects, non-intuitable objects. This latter group of objects I will refer to as intelligible.
Concepts of objects considered generally is not a category within experience, but an abstract category that belongs to critical philosophy. This is to say, that if we employ a concept it is either one we can represent or not and never indeterminate.
Look around you, and you'll see plenty of objects that are given through intuition that you have a concept for. No objects will be given through intuition that are sufficient to think of alone.
Now, if we assume a concept of God wherein He is unlimited we will find that He cannot be represented since this would require that God was restricted to space and time. Similarly if we take a concept like forgiveness we will be able to identify particular acts of forgiveness, but never any object that is adequate to forgiveness itself.
Importance in Kant's Work
The centrality of time and space as limits to our cognition cannot be overstated, and it is exactly these additional requirements of an object of experience allows us to draw a line a priori between objects we can give in intuition and those that we cannot.
As a result of this borderline, the transcendental logic has a point from which it can abstain from abstraction, and in this way distinguish it from general logic. Transcendental logic still considers the conditions for representation as necessary components of cognition, and so the categories themselves - being native to transcendental logic - are limited in their application to space and time.