See also: Offset 596d4n
The lead-in is the amount of time the game gives for players to react before the first hit object of a beatmap. This can be customised by mappers by editing the AudioLeadIn field in a beatmap difficulty's .osu file, which contains the lead-in duration in milliseconds.
The minimum lead-in time automatically used by osu! is 1.8 seconds. With the lowest approach rate of 0, this is the amount of time a hit object is visible before it needs to be hit. Any storyboard or video that plays before the first hit object may extend the lead-in time.
Using a custom lead-in is required by the ranking criteria in case the beatmap contains an epilepsy warning that covers the first few hit objects.