The Front Porch was the name that the Goblins of Goblin-town gave to a cave that opened onto the High Pass in the Misty Mountains. It was marked by a large rock standing in the path, behind which was a low arch and narrow opening into the mountain. The cave was just large enough to accommodate Thorin, the other dwarves, Bilbo Baggins, Gandalf, and all their ponies during the thunder-battle in the mountains.
The Goblin-town had a main gate in one of the passes of the Misty Mountains from which they assaulted travelers. As that way was eventually too dangerous and abandoned, travelers took the (seemingly safier) High Pass, so the goblins opened the Front Porch at some point before T.A. 2941 as an alternate means to snatch people.
Fíli and Kíli discovered the cave and reported it empty. Upon entry, Gandalf lit his wand and explored it thoroughly. Satisfied, the company dried out and relaxed before falling asleep. Fortunately, Bilbo had a nightmare about a crack opening in the wall of the cave and upon waking found that his dream was partially true. Goblins had already stolen the ponies through a real crack in the wall when Bilbo let out a very loud yell. The goblins then captured Bilbo and the Dwarves, but the yell had woken up Gandalf, who escaped with a lightning-like flash that killed several of the goblins.
 See also