Explain OPSEC???

I see a lot of people talking about OPSEC in their answers, some seem to know, some I think just use it to be stupid. I understand that it has to do with information pertaining to our troops and their moment. Is there a website I can look at to learn more about it? My fiance is in the Army and deployed overseas. When he was in Kuwait going to Iraq, he couldn't tell me when he was leaving or where he was going even though he knew, but now that he is there he can tell me where he is at? I guess it just doesn't make sense to me. Thanks for your help!