What is this connector symbol?
That is a receptacle for a barrel connector. The symbol is supposed to imply circuit functionality. The following is not a standard symbol (don't use it!), but it should demonstrate the connection:



When you insert the connector into the receptacle, the center of the connector contacts the wide pin (the rectangle). The body of the barrel pushes the bent piece of metal out of the way, which remains in contact with the barrel because of spring tension.

In addition, when the barrel is inserted it also breaks the connection between pins 1 and 2. This can be used to detect if the barrel is inserted.