This is probably a very personal aside from the author, Mark – describing himself as a young man in embarrassing personal circumstances.  Although it is unwise to try to join up Bible dots that are too widely spaced, there is some evidence to suggest that the Last Supper may have been held in Mark's family home (Acts 12:12) and that he rushed out in his linen sleeping cloth on hearing of a disturbance at Gethsemane.  The importance of this verse is to identify the writer as a participant (even if briefly and dishonourably), rather than merely reporting historical details which others observed.

The word 'fled' is significant.  It is the same word he used in the previous verse, "Then everyone deserted him and fled" (Mark 14:50).  In other words, the author of this gospel fully identified himself with the cowardice and faithlessness of the trainee apostles.  In this respect, there is no moral difference between him and Peter from whom he probably got most of his material.  The gospel he wrote was for people like him, weak people who run from danger, reluctant to commit themselves to the unknown and nervous about trusting Jesus.

Sympathetic, involved and eventually a convinced follower of the Lord Jesus, in Gethsemane this young man showed no greater loyalty than the disciples when it came to the test.  He too ran away, with nothing to hide either his modesty or his conscience.  However much this gospel urges us to see Jesus for who He really is, this fleeting act reveals that not even Mark understood at the time.  The 'hanging ending' of Mark 16:8 included the women as having no greater courage or spiritual insight: "Trembling and bewildered, the women went out and fled from the tomb. They said nothing to anyone, because they were afraid."   He and all of Jesus' followers needed the conviction and conversion by the Holy Spirit before they could be useful.

Running away from hardship and danger was part of Mark's character: as a trainee missionary he failed the probationary period (Acts 13:13, 15:37-39).  Even though Paul and Barnabas did not agree on whether he would be any use in the cause of the gospel, the Lord knew how He would use Mark - as a writer, rather than a pioneering missionary.  Mark's Gospel has discipleship failure written all over it, including Mark's.  That was why Jesus died - to pay for all that failure.  Mark's personal contribution to the sins of the world was etched upon the body of Jesus, and he knew it.  However, as Jesus said, those who know they have been forgiven much, love much (Luke 7:47).  Mark's experience of failure and forgiveness seems to have put great energy into his God-appointed ministry – as should every reminder of our own failure when covered by Christ's mercy and grace.