What does the phrase "it is up to us to flesh it out" mean?

What does the phrase "it is up to us to flesh it out" mean?

Can you suggest any synonyms?

From the New Oxford American Dictionary:

up to: the duty, responsibility, or choice of (someone): it was up to them to gauge the problem

flesh something out: add more details to something that exists only in a draft or outline form: the theorists have fleshed out a variety of scenarios.

So this sentence means (it may not fit exactly, you did not give much context): “it is our responsibility to complete this proposal”

"flesh it out" means to add flesh to the body of something. That is to say, fatten it up, make it more substantial, take a bare bones skeleton project/example and make it a fully formed entity.

In short it is using the metaphor of a body, a skeleton, that needs flesh to be complete - to refer to anything that is just the basic structure and needs to be filled in, completed, made whole.