Evaluation factors are the categories a proposal gets judged on, things like technical approach, past performance, management, and price, along with how they are weighted against each other. They usually live in section M of the solicitation, or a combined L and M section, and a strong proposal is organized to answer them directly, in the order they appear. Anything you write that does not map to a stated factor is not helping your score, no matter how good it sounds.

In a sentence

"Technical approach and past performance, when combined, are significantly more important than price." That sentence tells you exactly where to spend your writing time, and that being cheapest will not save a weak technical section.