ATL is an abbreviation for All-Time-Low, which is the opposite of an ATH (All-Time-High). It is the lowest point touched by a given coin. For bitcoin, the ATL was $0.00099. This was when bitcoin was just becoming freely tradable in 2009. It is also possible for a coin to hit its lowest point later. This is often the case with memecoins. These types of coins are often created and bought as a joke. When there is no more interest in these types of coins, they are sold and are worth nothing.