Non diegetic sounds are sounds that don’t naturally come from that scene. Examples of these sounds can be music that is playing but doesn’t have a source or a narrator talking. The sounds aren’t caused by anything happening in the scene. They aren’t something that you would hear in a particular scene such as the narrator talking or music playing with no source that it comes from.
Looking at the image above. There is no people around in the image so hearing someone’s voice talking would be a non-diegetic sound since there is nothing that would make that particular sound. Music can be another example of non-diegetic sounds since there is no radio or cars around in the scene so there is no source for it.
Movies tend to use non-diegetic sounds at the starting credits to try to get viewers into the movie before it properly starts and in some action scenes like preparing for battle or a competition. It keeps the viewers interested listening to music or narration helps the audience get an idea of what is going on/or help set the audience up for the events that are going to take place.