C++ primer 5 edition: Container of shared_ptr

Again reading C++ Primer 5 Edition. I am on chapter 12 Dynamic memory. Everything is OK. Until this point in the book:

"Because memory is not freed until the last shared_ptr goes away, it can be important to be sure that shared_ptrs don’t stay around after they are no longer needed.The program will execute correctly but may waste memory if you neglect to destroy shared_ptrs that the program does not need.One way that shared_ptrs might stay around after you need them is if you put shared_ptrs in a container and subsequently reorder the container so that you don’t need all the elements.You should be sure to erase shared_ptr elements once you no longer need those elements.

Note

If you put shared_ptrs in a container, and you subsequently need to use some, but not all, of the elements, remember to erase the elements you no longer need."

• I don't understand this paragraph can someone explain to me how may shared_ptrs leaks? and an example of the "container" of shared_ptr that can cause leak. Thank you.