What does it mean in The Handmaid's Tale when Offred says, "I am a national resource"?

Offred is referring to the fact that she is a handmaid and
therefore it is her role in this new society to be a fertile womb for a commander's
baby.  In this distopian society, there is a huge infertility problem, so in order to
maximize and control the pregnancies, the government has divided the women into various
roles -- marthas take care of homes, wives are married to commanders to maintain the
illusion of "normal" family life, and handmaids, in a "ceremony," have intercourse with
a commander in a highly ritualized and pseudo-religious way in the hopes of creating a
baby.  The handmaids are prized for their potential fertility, but they are also
subjected to the austere and emotionless "new order" of life in the Republic of Gilead. 
Offred remembers her old life and fully understands the implications of her situation if
she is unable to conceive and carry to term a healthy child.  She had a daughter in the
"old life," and therefore has proven her potential worth, but now must just make the
most of what little life offers, and now, therefore, she sees herself not as a mother or
wife, but as a natural resource to the Republic.