What is All-time high (ATH)?
The highest price a coin has ever traded at.
The percentage next to the ATH on coin pages shows how far the current price is below that peak. A coin 90% below its ATH would have to rise tenfold just to get back there.
ATHs are often set in short bursts of excitement and can take years to revisit, if ever. A large distance from the ATH can mean a bargain or a project that has lost its audience; it is not a signal on its own.