The management interface is for SSH/GUI access, sources RADIUS requests and things like that; the ap manager terminates the LWAPP/CAPWAP tunnel to the APs.
you will find much more information here:
http://www.cisco.com/en/US/docs/wireless/controller/7.0/configuration/guide/c70mint.html#wp1167723
Later controller modells (5500,2500) do not necessarily require an dedicated ap manager interface any more.
hth
Stefan
