"Evac", short for evacuate, is casting a transport spell to move a group out of danger to a safe point in the zone.
A druid has three spells that accomplish this: Lesser Succor (level 18 spell) or Succor (level 57 spell) or Exodus (level 59 AA). There are also items in the game, such as Crucible of Escape, that can accomplish this task. Inter-zone "Circle of" spells cast too slowly (24 seconds) to be useful for Evac purposes.
Succor takes 9 seconds to cast. Exodus is instant cast, but has a 72 minute refresh cycle. AA's can be used to reduce cast time of Succor, or reduce the refresh time on Exodus.
The tricky part of Evac is deciding when and if it should be used. You don't want to cast too early when the situation may be survivable. On the other hand, if you wait too long, most of or all of the group will be dead before Evac can save them. As a rule of thumb, if it is easy to return to your camp, then Evac early. If it is difficult or will take a long time to return, Evac late or not at all. Unfortunately, group (or raid) leaders usually don't have time to make the decision and ask for an Evac, so it is up to the druid to make the call himself.
It is generally recommended that druids get Exodus as one of their early choices for AA abilities.
The exodus and succor spells have become notorious for leaving group members behind, occasionally. The person left behind will see in red text, "The portal collapses before you can escape!" This can also affect the caster of the spell, with the caster rescuing the entire group, only to be left behind to die. Be prepared for any eventuality.