Need of a weak_ptr in C++11

I was reading "The C++ Standard Library" book by Nicolai M. Josuttis for understanding weak pointers. The author has mentioned 2 reasons for the need to have a weak_ptr and I don't get the second reason. Can anyone provide a simple explanation along with example of the below reason (quoted from the book) -

"Another example occurs when you explicitly want to share but not own an object. Thus, you have the semantics that the lifetime of a reference to an object outlives the object it refers to. Here, shared_ptrs would never release the object, and ordinary pointers might not notice that the object they refer to is not valid anymore, which introduces the risk of accessing released data."