When films were first shown there were no editing techniques. People marvelled at simple scenes of workers exiting a factory with no cuts in the filming or music playing. As audiences have become more discerning, editing has grown into a job in itself, cutting and sound are key to the language of modern films.
Understanding modern techniques is key if your audience is to follow what you mean with your edits. Watch lots of movies and examine how cuts are made. How do scenes end, what does a cut to black signify or a slow dissolve?
Editing styles can alter the feel of your film. Short snappy cuts give the film a tense, fast pace. Long gaps between cuts allow time for the audience to relax into the scenes.
Clever editing can create illusions, just the sound of a helicopter creates the illusion that there really is a helicopter just out of shot. Editing can also help smooth over problems, you can edit around a difficult interview by cutting out bits.
If you need to show more than one aspect of a scene, you can film the scene several times from different angles and in the edit cut between those angles.
Montage is the principle underlying all editing. The audience are trying to interpret your film as they watch, create meanings from the images and sounds you play them. If for example, you put together a politician’s speech without sound, followed by images of war, then the politician is assumed to be talking about war. Montage creates a new meaning from two independent images. Similarly you can lead the audience this way, show separate images of two people walking down a street and the audience will assume the two will eventually meet.