I read in my metric spaces book that a separable space is that which has a countable, dense subset. This definition has no intuitive meaning to me. I'm able to show if a space is dense or not, and I think I can show a space is countable. But, I'm missing the "so what?!" I would like to understand this concept better. Perhaps the root of the problem is my lack of understanding of the definition of dense. I know dense means the closure of the space is equal to the space (the space and its derived set). I know if a set A is dense in B, then for every element a of A, there exists a sequence bn in B such that bn approximates a (can't say bn converges to a since a not in B). Am I missing something about the concept of dense maybe? What sorts of fun things does one do with a separable space? What does a nonseparable space imply? Just a note - I am not familiar with topology, only real analysis and metric spaces. The way my classes were structured, we seemed to skip most concepts involving open sets , so if there's a way to help me understand without talking about open sets or open covers, that would be really appreciated.