This is first time I'm buying seconds so I don't really know. Here are a couple of comments from reddit:
How big a blemish we talking?
It means it's a shoe that has a defect that made it so that they could not sell it at full-price. Common examples are sloppy stitching, misaligned toe caps, or blemishes on the leather. They're often quite minor and pretty unnoticeable, and AE says that they should only have cosmetic defects, and not structural defects that will affect the life of the shoe.
Edit: Nevermind, found a few others that I liked and ordered. Let me give you guys a pro tip; order at least more than 1 quantity of the same pair so you can pick and choose which ones you want to keep and return the ones that don't suit your looks.
Example: Wanted the MCallisters (just one pair, but ordered 3 to pick which seconds has the least defects, then return the other 2 pairs) and did the same with 2 other shoes.