"yIHmey bopbogh paqmey'e' vIlaDQo'."
Translation:I refuse to read books about tribbles.
Without it, it would be ambiguous what is being relativised -- the subject or the object of bop.
It could be "I refuse to read books that are about tribbles" (relativising the subject, "books") or "I refuse to read the tribbles that the books are about" (relativising the object, "tribbles"). The -'e' makes it clear which of the two you are "taking out" of the clause paqmey bop yIHmey and using as the object of vIlaDQo'.
As another silly examples, consider 'ey chabmey vutta'bogh vutwI' -- that could mean "the pies that the cook has prepared are delicious" or "the cook who has prepared the pies is delicious". Using -'e' would ensure that there is no ambiguity.
Adding to this:
The -'e' is never actually required for disambiguating relative clauses. In fact, Marc Okrand rarely (if ever) uses this construct, but it is popular in the Klingon-speaking community.
It isn't really necessary in this case, because - as mizinamo says - the proper translation is quite obvious. However, the purpose of this particular lesson is to teach how -'e' can be used in this way.
When translating from English to Klingon, yIHmey bopbogh paqmey vIlaDQo'. is also accepted (along with about a dozen other variants).