This is a funny phrase. It is an idiomatic phrase that comes from Genesis. What distracted Eve? What got a hold of her focus? What got her off-track? Of course, the proverbial apple. The fruit of the tree of good and evil was the "Apple of her eye". This phrase is mostly used as a term of endearment as well. We can call our spouse the "apple of our eye", and of course, Jesus, our King, should be the apple of our eye. 

We're going to study two incidents this afternoon and wrap around them a parable that emphasizes the importance of what we are actively seeking at any moment in time in our lives. You see, there are things we can miss out on, just because we're not paying attention. 

YT: https://youtu.be/azVKLxyZf4c