Split it then I roll it then light it up like it's Independence Day I got a bottle rocket put it in the air
“Split it” means he is splitting a rello, or forming a blunt to smoke.
MGK also says that afterwards, he smoke its (“light it”) means that he’s dope (cool,nice,awesome, etc).When he light it, he compares himself to 4th of July (mainly towards fireworks and the smoke that surrounds the area).
“Bottle rocket in the air” could mean he’s high off of smoking or that he simply has a “bottle rocket in the air” (which could be champagne,beverage, etc.)
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