It's urban Black American slang. "Mad" is basically the new "radical" or "intense" as in: "The Deejay had some mad flow!"
Props is a shortened version of "propers", like when Aretha sings: "All I want is for you to give me my propers, when you get home". It means "proper respect" as in a level of acknowledged respect commensurate with one's social status, accomplishments or social caste within a given social construct. It makes respect a unit of social currency as opposed to a personal view of somebody or something.
Mad props = acknowledgement of one being due high regard.
---
I wanted my half in the middle and I wound up on the edge.