Watch your back inside the hall, new niggas slide through
Like doors yo, you're staring in the mess hall

from Raekwon (Ft. Ghostface Killah & Nas) – Verbal Intercourse Lyrics on Rap Genius

Meaning

He’s either:

Giving advice to the young blood about remaining alert in prison, you could be assaulted, extorted, raped, etc. by other inmates

Or telling himself to stay on alert of other inmates constantly coming through prison.

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