What exactly does "I want the best for you" mean?
When a mother says that to her child, what exactly does she mean?
Is it like a cliche that doesn't really mean anything?
I wouldn't say it was clearly not meaningless. It clearly expresses a wish, but the motivation behind the wish - and indeed the existence of the wish - is usually a matter for some doubt.
So a parent who says 'I want the best for my children' may mean anything between 'I'm a normal parent' (verging on meaningless) and 'I want my children to have and achieve and experience what's best for them'; their children usually think it means ' I want my children to do what I think will lead to the best outcomes for them'