What is it called when you filming a scene and voice narrating but its not narrator?

The term you are looking for is "voiceover". Voiceover is a production technique where a voice is added to a film, video, or other media after it has been recorded. It is often used to provide narration, commentary, or other spoken information that is not part of the dialogue between characters. Voiceover can be used for a variety of purposes, such as explaining the plot of a film, providing historical context, or adding humor or dramatic effect.