DIAGNOSIS You're looking at the heating element and fan shroud. Grease and food particles get splattered up there during cooking, and it does need occasional cleaning for performance and smell.
FIX The key is that the cooking chamber does come off for cleaning. 1) Unplug the unit and let it cool completely. 2) Open the door and grip the front of the black interior chamber—not the outer shell. 3) Pull it straight out towards you. It's held by friction and will come free. You can now wash this inner chamber in warm, soapy water (don't submerge the very top where the electronics are). Dry thoroughly before sliding it back in until it clicks into place.
IF THAT DIDN'T WORK For really stubborn, baked-on grease on the heating element itself (the metal coil), use a damp cloth or a soft brush (like a clean paintbrush) to gently wipe it while the unit is cool and unplugged. Never use abrasive pads or steel wool.
HEADS UP Never immerse the main base unit in water. You're only removing and washing the inner, black metal chamber. Putting it back in crooked can cause issues, so make sure it's seated evenly.