on Just Publishing Advice:

Cleft sentences use a two clause construction and usually start with the empty introductory subjects of what or it.

Cleft is an adjective that means to split or divide something into two parts.

But in grammar, we use this division to add emphasis to the first clause in a sentence. There are many different cleft constructions in English, but the most common are it-clefts and wh-clefts.

You may not use these often in your writing. Yet it is worth understanding how to correctly add more emphasis to certain sentences when the need arises.