Yes, they're idioms.
"Quiets your tears" does not mean to literally wipe away someone's tears. Though, I suppose, in the course of comforting someone it would not be out of place to wipe away any actual tears. The emphasis is more on "quieting" a person's sorrow - bringing them stillness, peace, etc.
To "zero in on" something means to focus on. I imagine this phrase takes its meaning from targets - imagine an archery target. The bullseye in the middle is the "zero" and when you "zero in on" something you focus on it, get to the heart of it.