It means "what are you doing", "what you gonna do today, or later on today. or the rest of the day."
Frank: "Yooo! wat u on tday bro?"
Ramon: "Shit, Just posted, bored af. Y was good?
Frank: "Ha, same lets hit up newport beach tday tho son."
Ramon:"Fasho, Im down, you just gotta drop on tha tank bro"