What does it mean for a flour to be "strong" or "reinforced"? When describing flour, I sometimes see people comparing flours and saying that one is strong or stronger than another. 

I also see flours that claim to be reinforced such as the Antimo Caputo 00 Rinforzato, where Rinforzato = Reinforced.

I had my own idea of what it meant, thinking it had something to do with higher gluten content, but right now, I'm not sure if that is correct.
Strong flour does mean higher gluten flour. I can't really find much info out about the Rinforzato flour. My guess would be that that could mean "enriched" but since another question (Red and blue Antimo Caputo 00 flour) seems to indicate that it has higher protein, it may just have some added vital wheat gluten to make it stronger.