Oui, j'en veux ...why en

 In French, "en" is a pronoun that replaces a previously mentioned noun introduced by a preposition, most commonly "de" (of/from). It helps avoid repetition and makes the sentence more concise.

Here are a few common situations where "en" is used:

Replacing "de" + noun: When the noun is introduced by the preposition "de", "en" is used to replace it.

Example: "Je veux du café." (I want some coffee.) -> "J'en veux." (I want some.)

Replacing phrases introduced by "de": When a phrase or clause is introduced by "de", "en" can replace the entire phrase.

Example: "Il parle souvent de son voyage en France." (He often talks about his trip to France.) -> "Il en parle souvent." (He talks about it often.)

Replacing expressions of quantity: When "de" is used to indicate an amount or quantity, "en" can replace the quantity.

Example: "J'ai acheté trois pommes." (I bought three apples.) -> "J'en ai acheté trois." (I bought three [of them].)

In your example, "j'en" is used to replace "du café" from the previous sentence. So "j'en veux" translates to "I want some [of it]."