Bonjour Phuc,

Le / la are direct object pronouns   

En is also a pronoun but not for objects, but rather in the sense "de cela" --> "about it / of that"

So the sentence makes reference to "un droit fondamental" and later in the sentence "il faut respecter "of it" or "about it"    making reference to what went before in the sentence namely  "La liberté d'expression est un droit fondamental" . This use of "en" removes the need to repeat the complete expression above --  using "en" as a pronoun serves to make this reference.

"La liberté d'expression est un droit fondamental mais il faut en respecter les limites."

https://www.lawlessfrench.com/grammar/en-adverbial-pronoun/

Hope this helps.

Bonne continuation

Jim

The sentence, before and after introducing en, reads:

Il faut respecter les limites de la liberté d'expression. --> Il faut en respecter les limites.

As you can see, the COI la liberté d'expression is introduced by de and hence a candidate for replacement by en.

Chris W, to me, the sentence is as such" Il faut respecter les limites de la liberté d'expression.  so respecter what?  les limites etc. so my question remains should we use en?