According to Kant, "Perception without conception is blind; conception without perception is empty." What does this mean?

This quote from Immanuel Kant is about the idea that we are an inextricable combination of thinking and sensory input. Neither sensory input nor concept can stand alone in a way that is meaningful or of use to us.

Imagine for a moment that all of your sensory perceptions have no way of being placed in a bigger picture. You see a beautiful child. End of thought. You smell some bread baking. End of thought. You hear a pleasing sonata. End of thought. And so on. Without being able to form concepts, we have no way of doing anything with our sensory perceptions. They just are. If we have concepts in our brains, we are able to organize our sensory input in a way that is useful to us.  For example, if we have the concept of danger in our brains, we can place our encounter with a snarling dog in that file and remember it for the future. We can make connections, like remembering that a sonata is by Mozart, so we should remember to listen for others by him. Concepts such as love, evil, democracy, or beauty are boxes in our brains that we put our sensory input in. Without these boxes, sensation has no past or future, only a fleeting and meaningless present. This is a kind of mind blindness.

Now imagine that you have all of these boxes in your brain, with no sensory input at all. All of these concepts would be mere words. We cannot think of anything but in concrete sensory terms. We cannot think of love without having had some tangible experience of it, whether our own or that we have heard or read from others. Danger is a concept that is built upon experiences, our own or others'. If that is not the case, it is a meaningless concept, unable to get our adrenaline levels up enough to respond appropriately, just a word. All that goes into our brains goes in as sensory input. It is the sensory input that fleshes out the meaning of the concept. Otherwise, concepts are empty boxes.