Amended returns?

My husband and I filed his tax return in January, before my W-2 came in. We asked the woman at Jackson Hewitt if we could file just his right now, doing married filing jointly and then come back later when I got my W-2 (needed the money from his tax return right away). She said that was fine, no problem. So we got my W-2 and went to base today (my husband's a Marine), and he told me that we couldn't file married filing jointly on mine because we had already used the married filing jointly with his, and I would end up owing taxes, instead of getting them back. So he said I had to wait until April 15th to file an "amended return"...what does that mean?? And he said something about filing an amended return for my husband too...I don't understand taxes at all, but if we do that, will we still owe taxes? What does an amended return do?