Drain time refers to the total duration of a beatmap that requires the player to be active. It begins at the first hit object, ends at the end of the last hit object, and does not include breaks. The word drain originates from osu! and osu!catch's health drain system, where the player's health is constantly decreasing as they play the map.
Beatmaps with very long drain times are typically referred to as marathons due to requiring endurance beyond the norm in order to be completed.
The beatmap info page on the website calculates drain time (labelled Drain length) based on only hit objects' start times, so if the last object of the map is a slider or spinner, the drain time will be displayed incorrectly.