S01E04 - what did Locke see?

At some point in the 4th episode of the 1st season, John Locke face The Monster (ofc we don't actually see it). But do we know what the Monster did/told/showed Locke in that moment? I've never understood it...

At some point in the 4th episode of the 1st season, John Locke face The Monster (ofc we don't actually see it). But do we know what the Monster did/told/showed Locke in that moment? I've never understood it...