You go up by either coming in first in ranked team games, or you come in the top half for Lone Wolves.
earn it.
Matchmaking
You must get to the rank Luitenant in a game type or for your rank.
rank up to brigadier and buy it for 300,000 credits.
you have to rank up to seargent
Get your overall rank to Lieutenant
I believe Lieutenant.
5 star general
it means to get a higer rank it doesnt make you better at halo 3 but what i said before it some times makes you a higher rank :)
Every time you win a match you improve your rank for that playlist, if you lose your rank may decrease, your rank in a ranked playlist will improve your overall status/rank(e.g as a sergeant or lieutenant), try it out, you start at level 1 rank so it won't decrease, if it increases like I said your overall rank may increase too.
It's call a 5 star General and it is the top rank in Halo 3
After the rank captain and captain grades 1, 2 and 3 is Major