Because opsom is a separable verb.
The dictionary form is opsom, meaning to summarize. In a main clause, Afrikaans often splits this type of verb:
Here:
This is very common in Afrikaans with separable verbs.
That is the normal word order for a separable verb in a main clause.
Afrikaans puts the finite verb early in the clause, and the separable part goes later:
So op at the end is not random; it belongs to the verb opsom.
Usually, yes: opsom often summarizes something.
For example:
In your sentence, the object is not stated directly. It is understood from context, and the second clause helps explain what is meant:
So the sentence is giving a general idea rather than spelling out the object in the first clause.
Yes. Met ander woorde is a fixed expression meaning in other words.
It is used when the speaker wants to restate or clarify what came before. In this sentence, the second clause explains the first one in a different way.
Intriges is the plural of intrige.
In this context, it means plots or plotlines in stories. A learner might first think of intrigue in the sense of secret schemes, and that meaning can exist too, but in a literary context intrige often means the plot of a story.
So here hulle beskryf intriges means that synopses describe story plots.
The semicolon links two closely related complete clauses:
The second clause explains or rephrases the first one, so the semicolon is a neat way to show a close connection.
A full stop would also be possible, but the semicolon keeps the two ideas more tightly linked. A comma would usually be too weak here because both parts are full clauses.