It literally means "loyalty and disavowal", which signifies loving and hating for the sake of God.
Sunnis were less enthusiastic about a concept that they associated with heterodox groups, and the founder of the Hanbali school, Ahmad ibn Hanbal (d. 241 H/ 855 CE) in particular was reported to have dismissed loyalty and disavowal to specific individuals as Bidʻah.
However within a few hundred years, the Hanbali scholar Ibn Taymiyyah (d. 728 H/ 1328 CE) adopted the idea in order to ensure Muslims were sticking on the straight path in disavowing the religious practices of the local non-Muslims.
Muhammad ibn Abd al-Wahhab (d. 1792 CE) being influenced by his teachings, taught this doctrine which subsequently has a central place in Wahhabism and modern Salafism.
This type of love requires one to defend God deen and to preserve it.