In The Hobbit what important feature on Thror's map does Gandalf point out to Thorin?

The most important feature on Thror's map that Gandalf points out to Thorin is the secret passage to the Lower Halls of Erebor.
When Gandalf first shows Thorin the map, the latter is skeptical. He believes that Smaug may already know about the secret entrance. Gandalf replies that the dragon may have knowledge of the passage, but there is little likelihood he can make use of it. 
It turns out that the passage is relatively small in size: it is only five feet high and only admits three people walking abreast at any one time. Gandalf points out that it is impossible for the dragon to crawl into a hole of that size. He then reminds Thorin that he also has the key to the secret door, which Gandalf gives to him for safekeeping.
For his part, Thorin is encouraged that the dwarves now have a surreptitious way to enter the ancient dwarf realm of Mount Erebor.