In a nut shell, reciprocity is the practice of a state recognizing a carry permit issued by another state that recognizes its own. With the amount of legislation being introduced about gun control, it's vital that we as gun owners and armed citizens stay on top of it. This means knowing exactly where it is legal or illegal for you to carry your firearm.
Now I live in the great state of Alabama (#WDE), gun laws here are looser than a $2 hoe's bra strap. If you want to carry, you can. No permit needed. Of course there are certain conditions that follow, but we can discuss that later. The problem arises when I go to other states. Everyone isn't as pro-gun as my place of residence. For example, if I take a quick trip to Atlanta, I'm good. My permit will be honored the same as it is in Alabama. But if I cross one more state line to the east, I'm putting myself in possible trouble. South Carolina, for some odd reason, does not recognize the Alabama carry permit. (Alabama recognizes a valid South Carolina carry permit though, weird...) To limit your chances of stumbling into a bad situation, I'm going to pass on one piece of valuable information, the NRA reciprocity map! This will answer all your questions about in which states you can or cannot carry.