The same thing you said, though in a more abstract way. By temporal I mean where they go on the timeline, and by semantic I mean the content of the sound. Much of the time what you hear simply matches what you see on the screen, either from production sound or from a sound effects library or by a foley artist, eg if you see a glass bottle smash on the floor you will probably just use a natural sound for that. But then you have to look at what it means in the context of the story - suppose the bottle is being smashed on the floor by an alcoholic who has finally decided to fight his addiction? You migh choose to emphasize that with music, or you might emphasize it by layering in other sounds from elsewhere in the soundtrack or some purely expressionistic sound like a heavy lock opening. Also, you'll frequently use sound to speed up or slow down the action by transitioning into the next scene before or after the camera cut.
Mixing involves setting the levels of the different sounds, but it also includes some decisions about how the sound moves, what sort of reverb and coloration are used (spatial) and how it's balanced against the music, how it's EQed (tonal).
Sound editors are responsible what to include in the track and when, sometimes much more or less than what the mics on set picked up.
Sound mixers are like sound engineers in the rest of the business, crafting relative levels, dynamics, tonal transformations like EQ, and spatial transformations like reverb.
What does that even mean? I thought sound editing was the creation of all the sounds and sound effects, and mixing was the mixing.