It's even a bit more complicated since the client has NA translations, but some people still use EU shortcuts.
MKK refers to the 3 dungeons in Lv.65 fortress in Upper Reshanta (Miren, Kysis, Krotan).
EOB (Engulfed Ophidian Bridge) is the 6:6 PvP battlefield. OB (Ophidian Bridge) is the PvE dungeon that is based on the same map.
Both IS & Kata refers to Infinity Shard (Katalamize for EU).
IO & RST is for Illuminary Obelisk (Rune Shield Tower for EU).
LDR is for Lucky Danuar Reliquary (Runadium Bonus for EU - I think some people may use Runa Bonus as well).
KB refers to Kamar Battlefield, which is a PvP 12:12 battle.
IWW stands for Iron Wall Warfront which is the 24:24 PvP battle.
EB stands for (The) Eternal Bastion. Some people may use SWB.
Both TF & TS refers to Tiamat Stronghold (Tiamat Fortress for EU).
And finally T-Dred is for Terath Dredgion.