Source selection is how an agency evaluates competing proposals and picks a winner under a negotiated procurement. A source selection authority, sometimes advised by a panel of evaluators, rates proposals against the factors published in the solicitation and documents the decision in writing. The two approaches you will see most are tradeoff, weighing quality against price, and lowest price technically acceptable, where price decides among proposals that clear a technical bar.

In a sentence

"Award will be made using a best-value tradeoff source selection process." That phrase tells you the evaluators are allowed to pay more for a stronger proposal, so price alone will not decide the winner.