The word Kafir (Hindi - काफ़िर, Arabic - كافر) is highly prevalent in islam. It is used for a person who does not believe in islam. As a result whosoever does not believe in islam is considered a kafir by muslims. However this is not the reality. A person who does low deeds, is mean, abject and of despicable nature is considered a Kafir.
Garib Das Ji Maharaj has given a vivid description of the word Kafir. He has described in detail about who is kafir.
Garib Das ji has spoken numerous banis which have been compiled in to a Granth Sahib. Garib Das ji uttered these banis after he met Supreme God Kabir Sahib. Kabir Sahib gave the knowledge to Garib Das Ji Maharaj in order to spread his own knowledge. The gist is that the bani of Garib Das Ji Maharaj is actually the knowledge given by Supreme God Kabir Sahib.