Gandalf is looking for someone to make a 14th person for the adventure. He wants a 'burglar' to help the dwarves.
That there is a burglar in residence looking for work.
He just uses it to write a sign on the door. It makes more sense if you continue to watch to when Thorin Oakenshield arrives and says "I would never have found this place but for the mark on the door".
Because the previous morning, Gandalf had "scratched a queer sign on the hobbit's beautiful green front door" as a sign so that the dwarves would know that Bilbo was the one he had chosen to go on their adventure with them. By knocking very hard on the mark with his staff, Gandalf put a dent in the wood over top of where the sign had been, erasing any evidence that it had been there. That way, Bilbo would not blame Gandalf for all of the dwarves coming to his house to take him on an adventure.
The raven
Elvish word for friend is mellon.
He places a couple of runes. They can be seen in one of Tolkien's illustrations. They basically say that a burglar is looking for work.
Bilbo was a young child and gandalf came thru with fireworks But he really met met him at his door and while he got the mail They both blew smoke rings remembering the fireworks and Bilbo inadvertantly wishing for adventure and times like when he was a young hobbit Bilbo good dayed gandalf several times in several different ways Gandalf marked his door after bilbo hastily departed after Inviting gandalf for tea the following day
After Bilbo shut the door on Gandalf, the wizard scratched some runes on the door. It was an indication that a burglar or treasure hunter lived there. Once Gandalf came for the unexpected party, he removed the mark.
They all look different They have different colored hoods.
Generally speaking, they don't trust them. The issue is that Elves are slow to decide and place little value on material wealth. Elves tend to rely a lot on one another, living very cimmunally, which is, essentially, the opposite of the Dwarves.
He left a mark on it he knocked so hard, it was this mark that told the dwarfs which door to go to.
He makes a sign on Bilbo's door that the dwarves interpret to mean "burglar for hire", which brings them into Bilbo's house.