Hinge propositions are those that are neither true nor false but instead cannot coherently be doubted by anyone. For example, it is not possible to ask "does reality exist?" without assuming that it does. Such propositions cannot be called true because it was never possible for them to be false.
In his On Certainty, Wittgenstein wrote about what came to be called hinge propositions as follows:
... the questions that we raise and our doubts depend upon the fact that some propositions are exempt from doubt, are as it were like hinges on which those turn.
That is to say, it belongs to the logic of our scientific investigations that certain things are in deed not doubted.
But it isn't that the situation is like this: We just can't investigate everything, and for that reason we are forced to rest content with assumption. If I want the door to turn, the hinges must stay put. (§341-3)