This is the second couplet of commands and promises from father to son in chapter three. “Love and faithfulness” are character qualities that dad said his son should always keep close to him (first command).
The second command, “bind them around your neck” (see also 6:21) would be understood like strands of a necklace. Love and faithfulness will make us much more attractive to others, like wearing a beautiful necklace. Being external, something seen on the outside, I would suggest connection to our actions.
“Write them on the tablet of your heart” is the third command. It suggests love and faithfulness should be a permanent part of my innermost being. On the inside, they will impact on my attitudes and thinking.
The overall direction from these three commands in clear. Father advises son to hook up closely with a couple of great qualities (“love and faithfulness”) and that will serve him well in both his actions (“neck” – outside) and attitudes (“heart” – inside).
The verse ends with a promise. These two qualities will empower our spiritual “(in the sight of God”) and social (“in the sight of man”) relationships. Sounds like pretty good outcomes. Sounds like a pretty good way to live!