Imaginatively she is of the highest importance; practically she is completely insignificant. (A Room of One's Own, chapter 3)
In this chapter, the speaker is grappling with the difference between the way women are portrayed in fiction and in real life. In fiction and poetry, women are portrayed as almost ethereal creatures, goddess-like. They are spiritual, beautiful, and perhaps most importantly, valuable. Fiction and poetry seem to praise women, even honor them.
In real life, in stark contrast to the beautiful depictions of women in fiction, women are under-appreciated, often mistreated, and even abused. They are often forced to marry based on the choices of their family or their social status, not their own volition, and they are expected to remain obedient to others, especially the men in their lives. Their lives are also often exhausting, and society expects nothing from women outside of obedience to men, housekeeping, and child-rearing.
Essentially, in fiction and poetry, women are praised and honored, but in real life, they are oppressed. The speaker creates a distinction between women in fiction and women in reality by stressing how, in fiction, women are of the "highest importance," almost divine, but in reality, they are "completely insignificant" to society.