AIX and most system commands consider memory 'used' if it is used by process or cache. Most systems under load will only have a couple 100 MB free ( this is tunable, check the vmo man page for details )
This will give a good summary:
nmon's memory display ( 'm' ) is probably the quickest and simplest way to get a quick overview. The rightmost section breaks it down into Process, System, Cached and Free, which is an oversimplification but probably all you need 95% of the time.
This is a good writeup of how AIX uses memory:
http://www.ibm.com/developerworks/aix/library/au-vmm/
Memory Utilization
