"Quand on tient à quelqu’un, on le retient." Découvrez la campagne #SécuritéRoutière #RetenirUnAmi avec @michelcymes
What is the meaning of the first sentence?
Tenir à qqun - to value, be attached to someone.
Retenir - to keep, remember
There are other possibilities, but without context it could mean "When you value someone, you remember him/it."
appart from the definitions explained and the meaning, this is a kind of joke usual in french:
to get a good slogan.