A searching question goes straight to the heart of the subject matter, possibly requiring an answer with a degree of honesty that the other person finds uncomfortable.
A casual way of saying to friends I'll see you again, sometime, (without a definite date or time having been set) - this is often abbreviated to 'Later' or 'Laters' as an alternative way of saying goodbye.
Shades of meaning is a phrase used to describe the small, subtle differences in meaning between similar words or phrases; 'kid' and 'youth' both refer to young people, but carry differing views and ideas about young people.
If something is a sight to behold, it means that seeing it is in some way special, either spectacularly beautiful or, equally, incredibly ugly or revolting, etc.