As the opening paragraph of this short story makes clear, this text concerns a search for identity by the anonymous narrator, a black man, who is struggling to work out who he is in a setting of racism and where lots of people have so many ideas of what it means to be a black man. Consider the following quote:
I was looking for myself and asking everyone except myself questions which I, and only I, could answer. It took me a long time and much painful boomeranging of my expectations to achieve a realization everyone else appears to have been born with: That I am nobody but myself. But first I had to discover that I am an invisible man!
This describes the sense of desperation of the narrator as he desperately tried to discover who he was, only having to realise at the end of the day that, tragically, the colour of his skin meant that who he was was actually unimportant and didn't matter. Others would only ever see him as a caricature and a stereotype of their own ideas of a black man. The narrator is therefore "invisible" because others are unable to see him for who he really is.