The Utter East was the geographical region at the most eastern edge of the flat, circular world of Narnia. It leads to Aslan's Country. On its west was the Silver Sea and on its east, beyond the solid wall of the sky, was Aslan's Country. It was a notable section of the End of the World, since it is from here that the sun would rise.
Utter East ended with the waters of the Silver Sea rising up into a standing wave, over thirty feet high, like a stationary yet watery barrier making a wall around the rim of the world. The sun rose each day from behind this "standing wave", which is why the sun appeared closer the farther east one was. The only land at the Utter Eastern End of the World was a small plain of green grass just slightly higher in elevation to the Silver Sea itself. Beyond this grass plain lay Aslan's Country which could be dreamily perceived atop a high mountain, visible beyond the other side of the glass-like Narnian sky, at the very exact edge of the world.
The Utter East was also marked by sweet - i.e. not salty, but drinkable - water. This fulfilled the prophecy which Reepicheep recited at the beginning of the voyage: "Where the waves grow sweet, doubt not Reepicheep, there is the Utter East." The sweetness of the water was first discovered by Reepicheep when he dove over the side of the Dawn Treader to answer the challenge of the king of the Sea People. It isn't clear exactly when the Dawn Treader entered the area of the sweet water
The "sweet" water was more than just not salty - it was described by those who drank it as "liquid light". It sustained the crew of the Dawn Treader so they needed to neither eat nor drink anything else, and enabled them to bear the light which grew increasingly intense as they drew near the eastern edge of the world. A few of the older members of the crew grew younger because of this water.
It was from the Utter East that the waters of Narnia rose significantly and then froze into solid ice.