The elves hold estel as the belief that Eru Ilúvatar--the Creator of the World that Is--is good, and his designs will end well for all his Children, despite our troubles. Estel is a belief that cannot be destroyed by the ways of the world or the suffering that we face therein, as we are the Eruchin, the Children of the One, and He will not suffer Himself to be deprived of us. Not by Himself, our own folly, or any Enemies.
So we hold to estel in even the darkest times and it keeps us from being overcome by the hardships we face in Arda Marred. Despite everything, the Children of Eru see the good in the world, alongside its corruptions and in spite of them. It is the Unmarred within the Marred, estel is the ability to discern the truth of it.
Have faith. It will be well in the end of all things, even unto the breaking of the world.