What does Low RDW-sd mean in a hematology test mean?

Means that the variation in the size of your red blood cell is very small. That it! Inother Words, no problem!
RDW refers to the variation in size if your red blood cells.

A low RDW means that the cells are all roughly the same size.

A high RDW means there isa high variation in the sizes of the cells and usually means a deficiency of iron and/or B12.