If you put crabs in a barrel to ensure your survival
You're gon' end up pulling down niggas that look just like you
What up, Blood? What up, cuz? It’s all black, I love us

from Kanye West (Ft. Jay-Z) – Murder to Excellence Lyrics on Rap Genius

The metaphor of crabs in a barrel could mean two things:
It may be used to mean people who are jealous of another’s success and thus try to pull him or her back down, the way that actual crabs in a barrel supposedly do when one crab climbs a little higher than the rest.

OR

He’s saying that where he’s from the environment is like a barrel full of crabs in the sense that everybody’s scrapping to survive, and in trying to survive you might have to pull some of your fellow crabs down. The crabs are meant to symbolize Black men in the inner city

Jay uses the 50 Cent line “What up, Blood? What up, cuz?” to demonstrate that he is talking to all young black folks (“Blood” refers to the Bloods gang, and “cuz” is a common Crip salutation), and to remind them not to get caught up in gang violence, but rather to feel pride in their race.

He may also be flipping 50’s line to take a shot at him for his recent attacks on Diddy and Rick Ross; Diddy called 50 a “crab” at the 2010 BET Awards.

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