Any reader (and any potential reader)

1) I absolutely agree, Rafee.   The title page tells what the report is about only to actual readers; it can't tell one who is a potential reader only, because he didn't read the title page and—   Oh, wait, I just realized; the author meant that the title page tells what the report is about to anyone who is about to read the report, and to anyone who is thinking about reading the report.   The latter is the "potential reader"; he reads the title page, and then, perhaps, decides that he doesn't need to read the report after all.   So I guess it sort of makes sense after all.

2) I judge the first one correct and the second incorrect.   Here's another example:

Q: What do you mean by "the end of the world"?
A: I mean the destruction of the earth after the day of judgement.

In both cases, "mean" is a transitive verb.   In the question, "what" is an interrogative pronoun; both that and (in the answer) "the destruction of the earth after the day of judgement" are direct objects of the verb.

I suppose your second question doesn't have to be wrong.   Maybe you have to ask which earth is being destroyed, or which day of judgement, and in that case you could say "I see what you mean by 'the end of the world', but which world did you mean it about?".   But that's stretching to find meaning in a question that normally would be considered wrongly put together.

On that "potential reader" question, I'd have to disagree. I think Bob's first impression is correct. If you are reading the title page, you're already a reader and no longer in the class of potential readers. It's sloppy writing imho.

If that's supposed to be some kind of helpful tip for report writers (or potential report writers) I'd say it like this:

1. Every report should have a title page. This tells potential readers what the report is about.

But then, why make it even that complex? Starting from scratch, I'd write the tip like this:

1. Every report should have a title page. This tells readers what the report is about.

Or even more simply:

1. Every report should have a title page that specifies the report's content.

'Sure about' and 'sure' each must follow its own grammatical structure:  

I am sure about   the color of the apple. I am sure it's red.
I am sure about   her nationality. I am sure it's Iran.

One is sure about   an attribute (color,nationality). One is sure the attribute is of a specific value (red, Iran)

Now what is the logical difference?   Clearly the sure about is implied in the sure and will become redundant once the latter is announced: no point in wondering about color if you are sure that it is red.

Now what about the negative form (or negative 'mood'?) as in Rafee's examples? There is no logical difference between not sure about and not sure– there is nothing the one says that the other has not already:

not sure about the color == not sure the color is red

So, Rafee's 2 examples, being negative, are really saying the same thing. One might tweak the wording some, but only to emphasize the grammatical difference, like for instance:

I'm not sure what exactly you mean by that.
I'm not sure about your meaning in what you just said there.

I think I'm going to end up disagreeing, Robert, but I'll have to come at it a different way.   Let's start here:   "I'm not sure it's Iran" is short for "I'm not sure that it's Iran", and therein lies a clue.   "I'm not sure about..." takes a noun as its object; "I'm not sure that..." has an object, too, but it's a verb phrase.   And when you look at it that way, there's no difference between the positive and negative forms.   Let's look at your examples again:

I am sure about the color [noun] of the apple.   I am sure it's red [verb phrase].
I am sure about her nationality [noun].   I am sure it's Iran [verb phrase].
One is sure about an attribute [noun].   One is sure the attribute is of a specific value [verb phrase].
One is not sure about the color [noun]; one is not sure the color is red [verb phrase].
I'm not sure what exactly you mean by that [verb phrase]
I'm not sure about your meaning [noun] in what you just said.

Having said all that, and despite it, I'm now reversing my original judgement.   Rafee, I still prefer "I'm not sure what you mean by that", but I think I was wrong to say that the other example is ungrammatical.   "I'm not sure about what you mean" sounds colloquial to my ear, but I don't think it's wrong.

I'm tempted to say that's because "what", when it's a relative pronoun, introduces a verb phrase but is itself a noun.   But this is a part of the English language that I'm fuzzy on.   What did they teach you about "what"?   Maybe you, looking in at it from the outside, can explain something I never understood.