This is the 2532nd most frequent French word.
celui
"Celui" means "the one" or "that one" (used as a demonstrative pronoun).
In this sentence, 'celui' is used as a pronoun meaning 'the one',
Celui qui travaille dur réussira.
The one who works hard will succeed.
Here, 'celui' specifies a choice among several options, referring to the blue-covered one.
Je préfère celui avec la couverture bleue.
I prefer the one with the blue cover.
In this case, 'celui' is used to indicate a specific object in proximity, emphasized by 'ci'.