In Tagore's poem, there is a call for absolute freedom, a demand for the reflection that is outside of the realm of limitation and constraint.  It is here where the images that Tagore employs help to bring out this freedom and sense of purity.  In the instance of "where knowledge is free," Tagore means to suggest that there can be a realm where comprehension and understanding is open to all.  Knowledge in its most pure fore is not to be limited or constrained.  It is here where I think that Tagore is at his most persuasive in constructing a vision in which there is unity and a sense of openness in being.  For Tagore, the idea of knowledge being "free" reflects how understanding and knowledge are elements that should not be restricted.  Feeding into the tone and theme of the poem, this becomes consistent with what other images are offered.  The freedom of the world from "domestic walls" or reason being set apart from the "desert" of limitation are both realities that help to drive the idea of what knowledge being free looks like.  Here, Tagore seeks to develop a transformational idea of being, one that seeks to create what can be from what is.