These are basic concepts in financial markets. It’s simple: We say “oversold” about an asset (in our case, a cryptocurrency) that is trading below “fair” value. This is what we call an undervalued coin that has fallen in price due to active selling.
We apply the concept of “overbought” to coins that, on the contrary, are obviously trading above their “fair” values. This happens when a cryptocurrency is overly bought and “undeservedly”—when some hype news goes public.