A second-string person is not on the main team, or in the main group, but is kept ready to step in when someone else cannot. The picture comes from sport: the first-string players start the game; the second-string sit on the bench and play if a starter is hurt, tired, or having a bad day. Coaches still train them, because a team needs that extra layer. People then carry the idea into work, clubs, and even friendship groups: a second-string reporter covers the story when the usual one is away; a second-string singer goes on if the star is ill. It is not a kind label. It says, without shouting, that you are useful but not first choice. In casual talk it can sound a little sharp, almost a put-down; in sport it is just a ranking, not an insult. It is about the job you hold in the group, not about whether you are a good person.