What does a low RDW mean on a blood test?

Means nothing. A high RDW is a measure of the variability in size among red cells. When it is low, it's a fluke of statistics, and you should ignore it -- lab "normals" are set so that a few healthy folks lie outside. I run a high RDW as a frequent blood donor, and elevation is a common first-hint that something may be unusual; many healthies have high rdw's for various reasons. Labs mean nothing in isolation.
This field is too sm. The red cell distribution width is a standardized way to communicate the size variability of a whole population of red cells. In 'normal' people there is a certain degree of variabililty. A population of abnormal red blood cells may have an alteration in the distribution of sizes of red blood cells. This measurement is not very specific or sensitive for any disease.
Low RDW is good. Rdw refers to the variation in the size of red blood cells. Normal value is from 11.5 to 14.5. There is preliminary evidence that high RDW values may portend higher mortality, however, this information has not become part of the accepted medical facts. The data for this association is based on information gathered by the insurance industry.