What is top-down processing as it regards to ESL teaching?

Question

as it regards teaching of grammar, words, comprehension

Answer 1

This is one of the two proposed schemes of how you can teach something; the other one is called bottom-up. I'll give you an example of them applied to the reading process.
Think of it as a pyramid; on the top-down approach you would start at the top of the pyramid, which is very narrow, by teaching the sounds of consonants like p, b, m, s... , you would then move to more complex sounds like vowel sounds, the next step would be to combine sounds in a correct way, after this you can start decoding letters and forming words, you would move to fluency, comprehension, etc. which would be the bottom of the pyramid.
The bottom-up approach is the opposite: it starts from the biggest point of view and then focuses on the smallest part.