The holy kiss does not have a lot of information or scholarship behind it.  And I wish I could be definitive about it’s meaning.  But I think the message behind this command is that we need to show camaraderie among believers.  We need to let everyone who comes into our fellowship to have a warm and welcoming greeting.  That no one should feel unwelcomed or shunned.  This is based on many of the previous one another commands that we have already looked at and is an extension the love we have received from our heavenly Father.  We are all a part of his family and so a friendly kiss or hug or handshake or pat on the back, or high five or other greetings should reflect this.  It is not just another handshake or hello.  It is a warm, loving embrace whether literal or implied.  So allow your greetings to display how much you care for each other.