What is instance attunement?
Instance attunement is basically where you have to do a quest or line of chain quests to be "attuned" to enter an instance. Without doing the quests, you simply cannot enter the instance portal.
This way of doing things is common in the earlier instances- it's something that Runewaker hasn't really done in Chapter 3, 4 or 5.
A lot of the level 50 and all of the 55 instances require an attunement chain. Let me see, here's my off the top of my head list (feel free to correct me Forum Folks):
Ice Dwarf Kingdom (level 35 and 50)
Cyclops Lair (50)
Treasure Trove (50)
The Origin (55)
Heart of the Ocean (55)
Hall of the Demon Lord and Zurhidon Stronghold (55- you get attuned for both on the same questline)
Cave of the Water Dragon (55)
Those ones have pretty long questlines that need to be completed before entry.
Some other instances require less work, but still have prerequisites you have to complete before you can access them.
The Windmill Basement (level 15)- need to get a Gloomy Vapor Stone from a Gulo in the Goblin Mines and talk to Uncle Kaga.
Arcane Chamber of Sathkur (level 15)- you can enter this instance anytime, but only after doing the attunement can you get access to the bosses room.
Mystic Altar (40)- iirc, does not require attunement, but you have to click on a series of statues to enter and progress inside.
Dungeon of Dalanis (57) don't need attunement, but doing a short questline enables you to transport easily to the instance.
Raksha Temple (60) do not need attunement, but you can't access the area by running to it without doing the questline.
So, ideally, you really want to do each and every quest available in game. It will make your instance life considerably easier, and as Camagic mentioned, each title you get adds 5hp. You also really need all that experience.
Remember that you have at least two classes that need to be doing these quests, and three if you choose to utilize the three class system.
Do them at your own pace, and if you run into trouble, the Runesdatabase is probably your best resource for quest help.