What does credit transfer really means?

I am considering entering LVN program and I am looking into schools and the best approach to completion. I read on this site that academic credits from some schools are not being accepted by others (not transferable).What does that really mean, can someone shed some lights?
If I want to pursue RN, would my diploma be useless, or I would need to take prereqs? I am confused!

also, how do I check if credits are not transferable? is that something that the school mus disclose?

thanks!