Wiktionary
a. (context idiomatic English) Not as good; of a lower quality or condition. n. (context sport English) A unit of players which is considered to have a secondary level of talent and which usually plays after the first string.
Wikipedia
Second String is a direct-to-TV film from 2002 about the Buffalo Bills football team who find its first string (led by real-life Bills quarterback Doug Flutie, who had left the team by the time the film was released) out for a month after a food poisoning incident, leading the team's head coach, " Chuck Dichter" (portrayed by Jon Voight), to hire an insurance salesman named Dan Heller (played by Gil Bellows) as the team's backup quarterback. Teri Polo also appeared as Heller's wife; Flutie, Mike Ditka, Chris Berman, Van Miller, Bills cornerback Donovan Greer and Ken "Pinto Ron" Johnson appear as themselves. The film originally aired on TNT.
Usage examples of "second string".
He said he was going out with the second string to jump them on the Heath.
Mackenzie duns, they're a lot smaller, about like his second string.
He was fascinated by the idea of there being a mysterious link between the woman and the animal, but he was already preparing a second string to his bow.