I'm guessing it stands for "street cred zero" as in having zero street credibility.

Wiktionary defines "street cred" as:

> Credibility among young, hip urban dwellers; particularly important in the hip-hop and rap scenes.

Google defines it somewhat more tersely as:

> acceptability among young black urban residents

Given that the user name says the user has no street cred, it's ironic that people would assume the user is "urban".