Learn Farsi: کارگران به این که حقوق کم است اعتراض کردند. - The workers protested the fact that the salary is low.

The verb اعتراض کردن (to protest) requires the preposition به to show what is being protested. In Farsi, you cannot attach a preposition directly to the conjunction که (that). To link the preposition to a full clause, you must insert the pronoun این (this). Literally, the structure means 'The workers protested to this, that the salary is low.'

Why is the verb in the clause است (is) instead of a subjunctive verb, since it comes after که?

We use the indicative mood (است) here because the clause states an established fact, not a desire, doubt, or hypothetical situation. The workers are protesting a reality that already exists (the salary is low). When این که introduces a concrete fact, the verb stays in the indicative.

Does the word حقوق usually mean 'salary' or 'rights'?

It means both! حقوق is originally an Arabic plural word meaning 'rights' (the singular is حق). In modern Farsi, it is the standard word for 'salary' or 'wages' (the money you have a right to receive). Context tells you which meaning is intended; here, being described as کم (low), it clearly means salary.

Could the main verb اعتراض کردند be placed before the به این که clause?

Yes. While this sentence uses a strict Subject-Object-Verb order by wrapping the main sentence around the clause, it is very common, especially in spoken Farsi, to move the main verb earlier to avoid a 'heavy' middle section. You will often hear: .