Meaning of “flat” - Learner’s Dictionary

flat

adverb uk us /flæt/ (flatter, flattest)

B2 in a horizontal or level position on a surface:

She spread the cloth flat across the kitchen table.
flat out

using all your energy or effort:

We've all been working flat out to finish the project on time.
in 5 minutes/30 seconds, etc flat

in exactly and only 5 minutes, 30 seconds, etc:

He showered and got dressed in 10 minutes flat.

(Definition of “flat adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)