The imagery being used here (Mark 13:36) is that of not being ready or prepared for Jesus when he returns. He warns us to watch for his coming and be ready because we do not know when he will return.
Jesus uses a number of parables like this to express the idea that we are to prepare ourselves for his return at any moment and watch for its coming. The warning is that if you are not ready you will miss out.
So what does it mean to be ready and watch as a Christian?
To me this means being and doing the things that Jesus would want us to do rather than simply doing what we want to do. In the Mark passage Jesus uses the image of servants being left to do the task assigned to them. The idea being that when the master comes back he will be happy with those who are doing the tasks he set and angry with those who aren’t (who are “sleeping”).
The tasks that Jesus has left us to is to make disciples of the nations (the great commission - Mat 28:16ff) and to live for God at every moment. This means reading his word and praying, meeting with other Christians to encourage and build them up. It means living a godly life and taking the time and effort to tell others of the hope and joy you have in knowing Jesus.
Not an easy task but with the help of God himself, we can do it.