If I read your post right, as in "What does ... mean", those three dots aren't parentheses; they're an ellipsis.
It's used in formal writing to show that something has been omitted from a quote, usually because the quote would otherwise be too long and the omitted material wasn't relevant anyway.
Another use is to indicate a pause in direct speed speech, as in "These ... are small. Those ... are far away.", or at the end of an incomplete sentence, eg "You don't mean ...?".
In cajar's example it's a placeholder for the answer so it's a special case of an incomplete sentence.
Another use is to indicate a pause in direct speed speech, as in "These ... are small. Those ... are far away.", or at the end of an incomplete sentence,