Lesson
This is one of my favourite phrases in spoken English. My dad used to tell this to me all the time. The basic meaning of the phrase is: "you need to face reality".
There are some large birds who put their heads in the sand when they believe danger is coming; like another animal trying to eat them. They can't see or hear when their head is in the sand, so they FEEL safe. Of course, the danger is still there, they just don't notice it.
If someone is scared to go to the doctor because they are afraid the doctor might tell them they are very sick, they "have their head in the sand" because they are afraid of what MIGHT be true. They are afraid of reality. This is a wonderful phrase and here is an amazing video that teaches this phrase and some similar ones. Enjoy :)