AM = "after market" Additional items that you can purchase from third party vendors to replace and/or enhance your kit. If you build without AM additions, then it is OOB (out of the box).
Some examples include resin parts (wheels, cockpit, bays) and photoetch details (PE); these AMs also require additional and more advanced techniques.
You can also replace/add decals, not sure if decals are considered "after market" in this context, as many times you need to purchase decals to complete some older kits in which the decals have deterioriated to uselessness.
Hope this helps!