Word means to applause some one on a good doing, to congratulate them.
"bruv big up Jerome, his lyrics are phat innit!"
"big up to all these homies in da hood, dis party is heavy"
when you are excited about something this word is uttered about 4/5 times.
"puh puh puh puh puh"
"fo shizzle ma nizzle , puh puh puh puh"