Kafir is an Arabic term (from the root K-F-R "to cover") "unbeliever", or "disbeliever". It is used as a derogatory term. The term alludes to a person who rejects or disbelieves in God and the teachings of the Islamic Prophet Muhammad, and denies the dominion and authority of God; or otherwise does not heed the beliefs and prescriptions held by the religion of Islam.
And Namaaz is ritual prayer prescribed by Islam.
So it's like prayer of non believer. Which do suits the story as both writer and army guy are person with conflicting morals and even Ex Army Men seems like non believer.
It metaphorically uses ‘Kaafir’ for impure and ‘Namaaz’ for pure – symbolizing the journey of its characters from a state of impurity to purity.
Even the title was one of the reasons for the controversy surrounding the release of film. That's why they decided to release it digitally.
Producer Bhargav Saikia claimed the movie's title was stuck with the producers' association since 2011. "Registration was refused apprehending political and religious controversies. We were asked to change the title since it was too sensitive. But we didn't," Saikia said.
According to Sharma, the title literally means nonbelievers' prayer. "Each character becomes vulnerable while sharing his darkest matters. With vulnerability, comes confession. I wanted to give a glimpse of this nature in the film's title while keeping the poetic gesture intact - from dark-to-grey-to-white