[ELI5] What does LOC mode do on a aircraft?
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[at least for most Boeing aircraft] Trying to avoid technical detail, that button effectively means “localizer guidance” from the autopilot. When pressed while the autopilot is already engaged, the lateral navigation mode will change to following the localizer (lateral navigation) of the Instrument Landing System frequency currently times. If not already on that path, the current lateral navigation mode will remain active until the plane intersects the localizer path.
In other words, “turn onto the path of the runway, and maintain that path”
When you want to land an airplane, you want to be lined up with the runway, and be moving straight towards it, so that you’re not fighting steering left and right while trying to go down. (you’re still doing some, but being lined up make it simpler/more accurate/safer). One way to describe is if you extended the runway, you’d want to be above the extended runway.
LOC mode uses a special radio to tell you how far you are off left and right, even if you can’t see the runway because of clouds or it being too far away to see. The autopilot can then “know” how much it needs to move left or right, and use that to steer the plane left and right to get the plane where it needs to be. A localizer only provides left-right directions, but not up-down directions.
The glide slope (G/S) provides the up-down navigation, but not left-right navigation.
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