My husbands mother called the other day and left a message saying she was going to sue us for grandparents rights, can she do that if me and my husband are married? And if she can what is it really?

In the U.S. anyone can sue anyone else. Whether the law will back them up is the real issue. Grandparents rights are relatively new to the family court system. They need to have a valid reason of concern for the welfare of your child to be given "rights" to the child. If they are just suing you because they don't feel they are being given access to your child, the courts don't really care about that. The child is not theirs and they have no right to force visitation. That being said, if they can afford a really good lawyer and are intent on doing this, they will make your life a living nightmare.

What is their reasoning for doing this?

By the way, this site should help. Scroll down to the middle of the page for State Visitation Laws. Grandparent Visitation