To be quite honest, when we say reported speech we mean exactly what it says!
Let's put it this way, we have two people, A and B. Now, let's assume that one of them, let it be A wants to inform you about what B said earlier when they were talking together. If we wanted to express this relation with a graph it would be like this:
B −( comunicate )→ A −( 'reported' comunicate )→ You
The reason reported speech is called this way is the fact that it's used when you want to report something, that another person said.
Let's try with an example, compare the following sentences:
direct speech: A: Nice weather, isn't it. B: Indeed.
reported speech: A said that the weather was nice.
For information how to form reported speech, you will need to ask somewhere else. You can also drop me a line and I'll answer for you if you needed that.