" Robert Louis Stevenson really like this quote because I think it very clearly illustrates the main problem that writing is concerned with -- communication. It is impossible to say anything -- let alone say it well -- if you do not know what you mean. I think that clarity of thought and clarity of words are completely united, and this quote is a very eloquent way of illustrating this connection concisely.

A also think that this quote neatly expresses how difficult it can be to fully communicate a thought. It is even possible that true and full communication is actually impossible. Because words mean slightly different things to each listener and/or reader, communicating exactly what you mean cannot really happen, because there is no totally common language in which to communicate. In literature, perhaps even more than in any other writing, being able to be as precise and exact...

At the same time, literature is always open to interpretation, which kind of exposes the irony of this quote. As important as it is to write what you mean as exactly and precisely as you can, it is impossible to let the reader know what that precise meaning is. The closer you can come to it, however, the closer everyone's interpretations of your writing will be to your intentions. This, I believe, marks the difference between literature that endures and writing that fades away -- when the meaning is too open to interpretation, there is little drive in the work to keep it as a part of the literature and culture that produced it.
Robert Louis Stevenson told wonderful stories, but he was also able to produce great clarity in his work alongside the excitement, and this is what made him great.