LOST means different things to different people.
But what is the word lost?
As an adjective, it means being unable to find one's way; not knowing one's whereabouts; unable to be found; (of a person) very confused or insecure or in great difficulties.
It also could mean something that has been taken away or cannot be recovered like an attempt recapture one's lost youth. Or an opportunity not used advantageously or wasted.
The word also means having perished or been destroyed such as a memorial to the crewmen lost at sea.
It could also mean a game or contest in which a defeat has been sustained by a player.
However, the origin of the word "lost" comes from Old English losian for ‘perish, destroy,’ also ‘become unable to find,’ from los ‘loss.’
The above denotes the various layers to LOST, the TV show.
First, the characters each had a backstory that showed them unable to find their own way through their lives. They were very confused or had great difficulties in their lives finding true happiness.
Second, many of the characters had lost something or someone in their lives that put them on a dark path of regret, anger or hopelessness.
Third, many characters wasted opportunities or friendships that led them down the path of loneliness.
Fourth, the main characters seemed to be both lost at sea and perished at the hands of the island guardian(s). Whether they were merely pawns in a game by the island powers is a plot debate point. But the word, as with the show, was about winners and losers in the struggle of power and conquest (the heart of business and personal relationships such as love).
Lastly, the origin definition may come the closest to telling what LOST was truly about: if you are unable to find (someone), you will perish and be destroyed by life.