It's part of a novel, so it has a larger context. The parable is not intended for you, the reader, but for the protagonist of the novel Josef K., who is spending time in a futile effort. I'd say it's basically about futility of seeking unattainable answers, and frustration. But it's probably not meant to be 100% understood, as Josef K. is a confused character full of doubts (like Kafka's characters tend to be), the purpose of the parable is not to dispel his doubts but to entrap him more in the frustration.