A business doing part of the work under contract to the prime, not to the government directly. Subcontracting is a legitimate way to get federal past performance and revenue before you are ready to bid prime, and large primes are often required to have subcontracting plans that send a share of the work to small businesses. The tradeoff is real: you rarely see the customer directly, and you are paid on the prime's schedule and terms, not the government's.

In a sentence

"The prime shall submit its subcontracting plan identifying small business participation goals." Those goals are your opening; primes with a plan to fill are actively looking for businesses like yours.