。
Dinner's almost ready.
Literal
Dinner-of preparations [topic-は] almost done-[ongoing-ています].
ほとんど ('almost, nearly, mostly') is an adverb of degree, useful for things that are near-complete but not quite. 準備 ('preparation, arrangements') is interchangeable with 支度 but slightly more formal. The state form 出来ています indicates an ongoing near-readiness.