Having poetic content; emotionally and/or intellectually. When it comes to rappers of substance, it means communicating something memorable that resonates because of how thoughtful it is or how much feeling/passion it has. Thoughtful doesn't have to be serious - it can be hilarious. Witty, etc. Big L was rarely ever 'serious' but he said a ton of poetic/funny/witty shyt that resonated.
A lot of new rappers have nothing to say - whether emotionally or intellectually.
Let's take J Cole for an example. You post a J Cole line - on paper it's dull as fukk. Meaning that intellectually, it doesn't resonate. The way Cole delivers the line (voice, cadence) is also dull as fukk. So emotionally it doesn't resonate. He's just a wack rapper. He doesn't make anything of substance, despite his stans saying otherwise.
Rocky makes music of substance. It's very passionate and creative. Does he say anything thoughtful? Almost never. "Demons" is probably the closest: "A rebel to my death - it's in my flesh it's in my vessels". What makes the line stand out in the song is the voice and cadence with which Rocky delivers it. It's passionate and adds to the feel of the song.
I hope that helps