ZEM is yet another EQ acronym, it stands for Zone Experience Modifier.
The ZEM is used in two ways:
Certain zones have higher ZEMs to make them more attractive as a way of balancing annoyance factors - out of the way locations, underwater, more difficult mobs etc. (Kedge Keep, Grieg's End, Acrylia Caverns, Veksar). Other zones have really high ZEMs because they are newbie zones - Befallen, Crushbone, Paludal Caverns, Kurn's Tower and The Warrens.
The ZEMs aren't static, not too long ago they announced that they were increased the XP modifier in some zones as an attempt to make them more popular.
What might be of note is that both The Grey and Lake of Ill Omen at least used to have negative ZEMs as the only zones...so if you're leveling up a new character you might want to skip LOIO.