commetary

The term most often used is probably narration. However, commentary could be used--it would depend on the subject of the film. If what is happening is self-evident, than anyone speaking in the background could reasonably be said to be providing commentary. If what is seen on the film needs explanation, than narrative is probably the better choice.

Commentary cannot reasonably be defined only as a spoken description of an event, and especially not limited to "while the event is happening." Commentary is frequently used in a literary context, to mean the remarks of an author about the work of another author. For example, it is quite common for christian writers to write commentaries on books of scripture. It is also common for authors to write commentaries on the works of ancient writers, particularly as modern readers may not understand many of the references in ancient works.