There is a block in every major city. It's a neighborhood where drug dealers are out 24 hours a day, 7 days a week. Basically, it's a literal drive through for illegal substances. You go, make your order, you make a block, and you pick up your stuff.
Killa: "Yo, you wanna hit up the block today?"
J: "Fo sho nigga, let's do this!"
What you do when you visit the block in your city. You make your order and they will tell you to "make a block". Basically, this means you drive around the block and when you come back, your order will be ready.
Killa: "Ey! Gimme four of 'dem boys!"
Citizen of the Block: "Aight, make a block"