En la reseña quiero explicar por qué el prólogo y la contraportada no explican bien la trama.

Spanish usually needs an article with a noun like this. En la reseña means in the review. English often drops articles more easily, but Spanish normally does not here.

It can also imply in my review if the context already makes that clear, even though mi is not stated.

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

• quiero = I want
• so yo is optional

You could say Yo quiero explicar... if you wanted emphasis or contrast, but it is not necessary.

After querer (to want), Spanish normally uses an infinitive:

• quiero explicar = I want to explain
• not quiero explico

This is the standard pattern:

• quiero leer
• quieres salir
• queremos hablar

Because here it means why, not because.

After verbs like explicar, saber, decir, preguntar, Spanish often uses an indirect question:

• explicar por qué... = explain why...

Compare:

• por qué = why
• porque = because
• el porqué = the reason
• por que = a less common sequence used in other structures

So in this sentence, por qué is correct because it introduces why.

It is acceptable in Spanish, even if English might sometimes prefer more variety.

• quiero explicar por qué... no explican...

This means:

• I want to explain why... do not explain...

The first explicar is about what the reviewer wants to do. The second explican is about what the prologue and back cover fail to do.

Spanish does not avoid repetition as strongly as English sometimes does.

Because the subject is compound:

• el prólogo y la contraportada

That means the prologue and the back cover, so there are two things, and the verb must be plural:

• explican = they explain

If there were only one subject, it would be singular:

• el prólogo no explica bien la trama

Because the nouns have different grammatical genders:

• el prólogo = masculine
• la contraportada = feminine

Spanish articles must match the gender of each noun.

Usually prólogo is prologue, but depending on context it can overlap with foreword, preface, or an introductory section before the main text.

In a book context, a learner should usually think:

• prólogo = prologue / introductory text at the beginning

The exact English word can vary a little depending on the kind of book.

Contraportada usually means the back cover of a book, or more specifically the text on the back cover.

In this sentence, it probably refers to the description/blurb on the back cover, not just the physical cardboard itself.

So the idea is that both the prologue and the back-cover text do a poor job of explaining the plot.

That is the normal position in Spanish. To make a verb negative, Spanish puts no directly before the conjugated verb:

• no explican = they do not explain

Compare:

• no entiendo
• no quiere
• no sabemos

Unlike English, Spanish does not use do/does in this kind of negative.

Bien is an adverb meaning well, and it modifies the verb explican:

• no explican bien la trama = they do not explain the plot well

So the meaning is not that the plot itself is good, but that the explanation is not done well.

The word order is natural in Spanish:

• explicar bien la trama

Spanish often uses the definite article where English may or may not.

Here la trama means the plot, referring to the specific plot of the book being reviewed. Since that plot is understood from context, Spanish uses the article naturally.

So:

• la trama = the plot / the storyline

Yes, depending on context, it can suggest something close to describe, present, or summarize the plot clearly enough.

It does not necessarily mean a full, detailed explanation. In a review context, the idea is more that the prologue and back-cover text do not give a good account of what the story is about.