Learn Portuguese (Portugal): Aqueles parafusos é que são bastante pesados. - Those screws are the ones that are quite heavy.

In this emphatic structure, é que acts as a fixed, invariable phrase that simply adds focus to the words right before it. It never changes to match a plural subject like Aqueles parafusos.

How does adding é que change the exact meaning here?

Without é que, the sentence is a simple fact: Aqueles parafusos são bastante pesados (Those screws are quite heavy). Inserting é que emphasizes the subject, giving it the feeling of "Those screws are the ones that are quite heavy" (as opposed to some other objects).

Why is the second verb são plural?

Even though the fixed phrase é que sits in the middle, the main verb of the sentence still has to agree with the subject. Since the subject Aqueles parafusos is plural, the main verb must be the plural são.

Why isn't bastante pluralized to match pesados?

In this sentence, bastante is an adverb meaning "quite" or "very" and modifies the adjective pesados. Adverbs do not have plural forms. If it were an adjective describing a noun (like "enough screws"), it would take the plural form bastantes.