Jesus surprised His audience by describing a scenario in which a homeowner, instead of being served by his servants, puts on an apron and serves them. This was completely unconventional for that time, but it reminds us of Jesus washing His Apostles' feet. Jesus combined the roles of master and servant in a unique way, which was not the norm.
The homeowner's act of serving his servants is a response to their faithful vigilance. The Lord who sustains us expects us to be faithful and vigilant so that we are ready to welcome Him whenever He arrives.
In the Book of Revelation, the Risen Lord says, "Behold, I stand at the door and knock." Jesus is always knocking at our door. If we welcome His daily coming, He will serve and support us in countless ways.