This statement encapsulates much of Kant's thinking about the relationship between sense impressions and thoughts found in the Critique of Pure Reason.
Our intuitions, are the representations or perceptions which are given to us of objects. They are not noumena, or things-in-themselves, which are inaccessible to us but rather phenomena as we reflect on them in our minds, filtered through the fundamental frameworks of space, time, and causation which structure our perceptions. Without concepts, however, they are random and meaningless. For a person to understand a brown blob passing across her visual field as a bird, and understand it as a warm-blooded feathered creature, requires having concepts of flying, birds, living creatures, etc.
On the other hand, a concept of a bird is empty without experiences of birds. In fact, one cannot really conceptualize birds without having experienced many different birds and begun to understand their commonalities.
In other words, we can only know or understand the experience of a bird flying past us through a combination of perception and a priori knowledge.