In SharePoint `PreSaveItem` function is called when `onClick` event handler is fired for Save button in List Forms.

`PreSaveItem` function:

    function PreSaveItem()
    {   
       if ("function"==typeof(PreSaveAction))
       {
    	  return PreSaveAction();
       }
       return true;
    }

`PreSaveAction` function allows to override behavior for a Save button. It is commonly used for client side validation, for example:

    function PreSaveAction() {   
       return ifValidSSN();
    }