Time Capsule Interface List
gec0 and vlan1 are the most important for monitoring overall traffic flow. 

gec0 corresponds to the local traffic (behind the firewall) and vlan1 represents external traffic upstream to the internet. I haven't seen anything that makes sense what gec0 but on larger cisco switches they were internal routing queues where packets went to get headers constructed and routed across Vlans in the switching fabric. Perhaps it's the same on airport hardware.

athX are wireless vlans
wlanX are wired vlans

Apple used to publish MIBs for SNMP back when the first Airport Extremes were around but I don't know of anything better than using a generic MIB for the more recent hardware.

There used to be more information on these ports up at http://www.uname.nl/closer-look-airport-extreme/ but I can't find it anywhere (since archive.org respected it's robots.txt directive) - the best is a dated summary from http://gigo.com/archives/blog/monitoring-apples-80211n-airpo.html