Learn English – What does this structure “bully somebody into something/into doing something” mean

In many cases, there's not really a reason why we use one preposition rather than another. In this case, though, I'd say we use from because it conveys a sense of away, apart, separate (you're keeping the person and the action "apart").

But note that we don't need to use a preposition at all - "She sang a lullaby and stopped the baby crying" is fine (as is "...stopped the baby from crying"). Also note that in related constructions like "She helped the baby to eat", "to" isn't a preposition as such - it's just the "marker" for the infinitive verb form "to eat".

And while we're on the subject, I'll also say that in "She sang the baby to sleep", the preposition "to" seems quite "rational" to me because it reflects "movement" from wakefulness to, towards sleep.

You can say

What do you mean?

to ask someone to clarify something they said. Or

What is the meaning of this?

to express outrage at someone's actions or a situation. Or

What is the mean of the data?

to ask about statistics.

Edit

For the ring-in-the-ice-cream scenario, you might ask,

What does this mean?

But really, a ring in your ice cream is so surprising you might simply ask,

What is this?