Instead you'll play Elliott, a brand new passenger on Oceanic Flight 815, with his adventures on the island dovetailing with the major events and most memorable moments from the show. It's up to you to work out where you are in the game in relation to the series, since the timeline isn't always immediately apparent. So, in rough order, you can expect to play some part in the first appearance of the Black Smoke monster; Ethan's deception; the unearthing of the hatch; Boone's death; the building of the raft; the appearance of the Others; the finding of the Black Rock ship; the introduction of Desmond and the tail section survivors; Shannon's death; the capture of Henry Gale (Ben); the discovery of a second station; the deaths of Ana-Lucia and Libby, and no doubt many, many more twists and turns we've forgotten about. You just never know with Lost.
Elliott definitely doesn't have a clue what's going on, because the plane crash leaves him suffering from amnesia - a convenient plot device designed to make him more believable since all the weird stuff you find both on the island and about his character is what he's discovering for the first time too. Interestingly, how Elliott acts on the island is completely different to his personality in the flashbacks.