1. What does "a false ceiling" mean?

2. Some times people start their sentences with a word like "Essentially , a computer is a machine".

What does "essentially" mean here?
1. An item that looks like the ceiling, but is not the ceiling. Possibly a place to hide things.

2. The essence. The basic quality. "As intelligent as computers can seem to be, essentially, they are just machines."
A false ceiling is merely a second lower ceiling that has been put in under the real one, f a room is too high for some reason. You most commonly find them in offices (above the false ceiling is loads of cabling and wiring and pipes and things, all attached to the 'real' ceiling, and the false ceiling is put in to make the office look smarter.