F.O.D stands for Foreign Object Damage.
If you see a photo from the front of a MiG-29 (I think this is also true for Su-27s) as it lines up un the runway for take off, you'll notice that the intakes appear to be blocked off. Those blockages are F.O.D. screens to keep the engines from sucking up any debris on the ground and getting damaged by it.
Once the aircraft is up of the ground, those screens retract flush against the roof of the intake duct, they return to the down position at landing.
When the F.O.D screens are down, theres a series of auxiliary air inlets on the top side of the wing leading edge extensions, just aft of the cockpit, that open up to compensate for the reduced airflow directly into the intakes at take offs and landings due to the screens being in place.
Hope that clears it up for you.