What are these numbers visible from the runway at Narita airport?
A good way to answer questions like this is to look at the Aerodrome Chart

As newmanth answered, these are gate numbers.



Using Google Maps, you can also see these numbers painted on the taxiway as a guide to taxiing pilots





If you want to understand airport air-side signage, one document I found useful is CAP 637 Visual Aids Handbook. It is a UK CAA document but I expect most of it applies in other parts of the world.

The CAA refer to stand numbers, as far as I know these are the same as gate numbers. They indicate parking positions for aircraft at airport terminals, these are places that allow for the loading and unloading of passengers.

Airliners arriving at airports will know what gate they have to taxi to, and ATC will provide them with taxiway routes. Airports provide signs so that pilots basically know where to find their allocated gate.