Cleft means "divided". In cleft sentences, one sentence is divided into two parts, each with its own verb.
This adds emphasis to part of the sentence.
John loves Mary. (one verb)
It's Mary that John loves. (two verbs, emphasises Mary).

The following structures are commonly used to begin cleft sentences:
It + .... 
It was a .... who..

It was in... that...

What +...

What I like about ... is

What they didn't realise was...

Other structures:
The person who...
The place that...
The thing that ... is/was...
Something that... is/was...
The reason why... is/was...
the only thing that ... is/was...
All that I would ... is/was...

We can use Wh- words with cleft sentences. To emphasise the actions, we use a form of do.
Make invested well. = What Mike did was invested well.


emphasising with What, All and It
To emphasise an action or series of actions, we can use sentences beginning with What.
He dropped the vase. What happened was (that) he dropped the vase.

We can use Wh- clauses as introductory phrases.
What I would like to know is where the money went.

We can use all instead of what.
All I'd like to say is that the company appreciates your work.

We can use It + be + that/who to emphasise parts of a sentence.
Mike left his bag on the train. = It was Mike who left his bag on the train. (emphasis on Mike) = It was his bag that Mike left on the train. (emphasis on bag).

It's because you have such a good sense of humor that I enjoy your company. (emphasising reason).

It was only yesterday that I discovered the documents were missing. (emphasising time).

It was by chance that they met in Paris. (emphasising prepositional phrase).