Gandalf is going to Rivendell, the home of the elves.
Bilbo and company are on a journey to take back the dwarves treasure from the dragon. When the travelers are done fighting the trolls, they continue on their journey. Gandalf has not told them exactly where they are going, so they ask him.
"You are come to the very edge of the Wild, as some of you may know. Hidden somewhere ahead of us is the fair valley of Rivendell where Elrond lives in the Last Homely House. I sent a message by my friends, and we are expected.” (Ch. 3)
Rivendell is the home of the elves. It is a very special place, and well hidden. There they can take shelter, be protected, and get some food and rest. They also need advice from Elrond, the king of the elves.
Bilbo has never been to Rivendell and has never seen elves, although he has long wanted to. He finds Rivendell a literally enchanting place.
Bilbo never forgot the way they slithered and slipped in the dusk down the steep zig-zag path into the secret valley of Rivendell. The air grew warmer as they got lower, and the smell of the pine-trees made him drowsy… (Ch. 3)
Once there, they can get advice on how to get inside the Lonely Mountain. Elrond helps them interpret the runes on the map. It turns out they say that they can get into the mountain when the thrush knocks and “the last light of Durin's Day will shine upon the key-hole.”
This turns out to be the key to getting into the mountain, but the travelers still have trouble getting in. They are about to give up when the moonlight shines on the keyhole and they are able to enter.