French language Q&A Forum

Searching through Google I came across Lawless pieces on variable and invariable pronouns. I'm still not sure how my use of Personne was wrong, or how aucun can be either an adjective or a pronoun, but I can live with that expecting a glimmer eventually, but it would be helpful if you could explain the terminology. Why are they called variable and invariable negative pronouns? Is it because the invariiable ones don't agree, whilst the variable ones do? This is one of those things people who know this stuff take for granted.

How would "what's that (used) for" and "what does that mean" look in french (considering the three ways of asking question(s))?

1) À quoi ça sert? Que veut dire ça?

2) Ça sert à quoi? Ça veut dire quoi, ça?

3) Qu'est-ce que ça sert? Qu'est-ce que ça veut dire?