Thanks for explaining that to
@the AntiPusher . In addition AP, the qualifiers have to win 3 matches to earn their spot in a major. AS MM said, LL are players who won 2 qualifying matches but lost the 3rd one. These players hang around the start of a major to see if someone will withdraw before playing R1. If a player withdraws before playing their first match, the tourney goes back to all the players who lost in the final round of qualies. They basically pull names from a hat and inserts that player in the withdrawn spot (why it's called Lucky Loser). They lost in qualies but were chosen and awarded (lucky) an opportunity to play in the main draw.
Sometimes, the LL's won't hang around if a smaller tourney or a challenger is starting up. Sometimes they prefer to travel to the other tourney. If that happens and their name is called to replace a withdrawal, the tourney will ask if they are still on the premises. If they aren't, they concede their spot and the officials pull another name from the hat.