The ATH is the highest price an asset has ever recorded in its trading history. In the crypto world it's a frequently cited benchmark, since many assets have gone through very sharp boom-and-bust cycles.
"Buying at the ATH"
It's often used as a warning: buying right when an asset hits its all-time high carries the risk that, if the price never surpasses that level again, any later purchase stays underwater until a new high is reached (which can take months, years, or may never happen).
A new ATH doesn't automatically mean overvaluation
An asset reaching a new all-time high doesn't by itself mean it's "expensive" or "cheap": that depends on the asset's fundamentals, not just its price chart. Even so, it's worth analyzing the context (DYOR) before investing just because an asset is at its highs.