What does it mean when a product is on 'back order'?

So, I went to the eye doctor and had an exam and also my eyes dilated. This was two weeks ago. And I ordered some contact lenses because all the eye glasses fall off my nose no matter what kind. So I have used contacts before but it has been a while since I haven't. My eyes are both -5.50 in the prescription. The lady there did not give me any trials and charged me 54.00 plus tax for each box. Well two days ago, she called me and told she had to charge me more for the boxes. Its acuvue for astigmatism. I went in again and paid. Today she called again saying that the contacts were on back order. I asked her why. She told me that she didn't know for how long they were going to be on back order. That it could be a week, a month a year, ten years. So I asked for my money back and she said impossible. But what does back order mean?

Read more: What does it mean when a product is on 'back order'?