Available Seat Miles (ASM) is equal to the number of available seats (whether occupied or not) multiplied by the amount of miles traveled.
For example, an aircraft with 100 passenger seats that travels 500 miles creates 50,000 ASMs. This metric represents the capacity of an airline- the more ASMs, the more capacity the airline can sell. Higher ASMs signal airline growth as capacity is added through increasing Fleet Size and/or new routes served. However, when travel slumps and demand for air travel drops, airlines often cut ASMs to reduce their operating expenses.