"Once in a blue moon" you may come across sayings where the meaning is not at all obvious. "Rest assured," it is probably just an idiom, a phrase where the meaning cannot be deducted from the actual words. In English, there are many, such as "straight from the horse's mouth," or "raining cats and dogs." However, every language is full of them.
Indeed, looking at different idioms can give you an insight into the culture of a language, and some good laughs as well. If you would like to learn some of these literary curiosities, "don't miss the boat," and take a look through this gallery.