Tried to add explanation but post edit time ran out
An aftermarket car stereo generally has 2 power inputs.
Constant Live and Switched Live. The constant live is to keep all your settings, etc. The switched is to turn the stereo on only when the ignition is on (stop the stereo draining the battery)
Some factory head units rely on an electronic system to tell them if the ignition is on or not so do not need the switched live. This is why the harness comes with a red switched live wire as your aftermarket stereo will not be able to use that system.
Its not hard to wire to the fuse panel.