This is when one’s love of something or of a sin makes him commit kufr such as abandoning Salah or hating why this sin is prohibited or he may falsify what the Prophet salla allahu alaihi wa sallam had said due to his love of that sin.
Committing a sin while believing it is prohibited is fisq and a sin in itself but not kufr!

Shaykh you have told me that loving a sin isn’t kufr and wanting to please your desires isn’t kufr. So what is shirk of love? You have said it is when you love something more than Allah. I worry that I don’t love to please Allah because I unfortunately love my desires and it leads me to sometimes sin. I ask for forgiveness.