Pregnancy week 21 urinalysis show specific gravity of 1.01 normal range (1.003-1.030) everything else is perfect what does this mean?

Well hydrated... Specific gravity is essentially a measure of how concentrated your urine is. If you are dehydrated, the number goes up. If you drink lots of water, the number goes down. A slightly low specific gravity just means you are well hydrated. Good job! Its nothing to worry about.