The Silver Chair is a quest story in which Aslan, the great Lion, gives two childen, Eustace Scrubb and Jill Pole, four signs to help them find a long lost and only son of a king.
The importance of the signs is twofold. Firstly, the signs indicate that Aslan does not expect the children and their Narnian guide, Puddleglum, to find the lost prince on their own; and they will prove to whoever is to give help that Aslan has sent them - otherwise anyone can say that they have spoken to the great Lion. Who's to know if that's true.
At first these signs may look cryptic and deliberately obtuse. However, when the children see with hindsight how they have missed a sign, they realise it is only their own distraction with the circumstances they are in, or with annoyance at each other, that has blinded them.
Secondly, although the following of all the signs is required by Aslan, the ommission of one will not compromise the quest. It is possible that the following of any one sign will resolve the story and bring about the release of the prince. This is indicated in the observation of Glimfeather the owl, who says that if Eustace had spoken to Caspian, an old friend, as instructed by the first sign, then the old king would have given them an army to help them. In other words they would have had an easy success. This 'redundancy' built into the number of the signs reveals the generosity - and the mercy - of Aslan: he does not set puzzles to deliberately fog the children.
Nevertheless by 'muffing' each sign, the children and Puddleglum make their task increasingly more dangerous. The immeadiate danger is to their souls. The underlying theme in this story is that the temptation to unbelief is extremely strong. This is shown in the case of Jill who, despite speaking to Aslan face-to-face on his high mountain (like Moses), is tempted by the Green Witch to say that there is no Narnia. She is strongly tempted because she, along with the others, has fallen into the Witch's kingdom of Underland - the very opposite of Aslans' high mountain, where "the air is clear and your mind is clear". The blandishments of the Green Witch are all intellectual. This is the wisdom that is 'earthly, unspiritual, devilish'. The witch has a plan for the prince and the Under-Earthmen (and here Lewis is thinking of 1 Samuel 15:23).
In the Underland there is that "vile engine of scorcery", the Silver Chair.
This is the device that restrains whomsover should sit in it, when they have, momentarily, come to their right mind and remember Aslan and that they are enslaved in a flat, skyless world. The sitter, even in waking, is powerless escape the chair. The children and Puddleglum must risk the possibility of death in following the fourth sign, revealed to them by the very captive in the chair. Is he murderously mad? Or is he their last hope of release from the Witch's domain? Can Jill remember Aslan? Has Aslan forseen that the following of the fourth sign will save both the savers and the captive? Is he right? Can we trust Him?