A second string is the backup to your primary booty call person. Usually unwanted, they'll make do when the primary person is not around.
Fundo's booty call girl was busy sucking his friend off, so Fundo called up his second string hoe.
A fat desperate guy that serves as back up to the main man when you're bored. Someone just to hang around to take up ones time. Lacks all the qualities of your main guy but is sufficient if marginal company when you're bored. Someone who you really don't care about.
That fat guy you were talking to is second string to the real deal.