We can combine two or more simple sentences into a complex sentence by using the joining word that. A complex sentence has one main clause and one or more subordinate clauses. The subordinate clause could be a noun clause, an adjective clause or an adverb clause.

The joining word that can introduce subordinate clauses of all kinds.

In the following sentences that introduces a noun clause.

We stand for peace. That should be known to everybody.
That we stand for peace should be known to everybody. (Here the connecting word that introduces the noun clause ‘that we stand for peace’.)

He would be punished. I told him that.
I told him that he would be punished. (Here that introduces the noun clause ‘that he would be punished’.)

That can also introduce adjective and adverb clauses. Study the following sentences.

This is an old fort. It was built by Akbar.
This is an old fort that was built by Akbar. (Here the connecting word that introduces the adjective clause ‘that was built by Akbar’.)

This is a fine opportunity. It should not be lost.
This is a fine opportunity that should not be lost. (Here that introduces the adjective clause ‘that should not be lost’.)

Our soldiers fought so bravely that the enemy retreated in disarray. (Here that introduces the adverb clause ‘that the enemy retreated in disarray’.)