"Follow me" is the call of Jesus. To be a disciple of Jesus is to be called.
Because the concept is somewhat "fluffy", it is easily missed by the modern Christian, but only to his great peril. To miss calling is to miss Christ, because we are only followers of Christ when we in fact follow Christ - that is, when we leave all other allegiances and walk after him.
Calling is part invitation. Invitation because it is optional. A human being must accept it in order to answer it. Disciples are accepting Christ's invitation.
Calling is part challenge. Challenge because it is different. A human being must elevate to new standards of thinking and being in order to answer it. Disciples are elevating to new standards of thinking and being.
Calling is part command. Command because it is consequential. A human being must become what is being demanded in order to receive the promised rewards. Disciples are obeying and receiving the Caller's rewards.
Disciples of Christ are easy to spot. They act daily and oftentimes spontaneously out of a deep center. There is an authority that governs not just what they do, but how they do it.
That authority is called Calling.