A template default is a set of parameters that you have chosen for an atom, that you wish to be applied to every subsequent page created from that template.
I have created my first web page and on that page I have a menu. I have configured the menu as I want it to appear on every page...
This is simple to do, however you would not want to do it more than once, as this would be very time consuming. Therefore when you've set it as you'd like and saved it, you would right-click the atom and choose to "Push To Template Atom Default". This then makes it the default content for that atom whenever a new page is created.
Pulling basically means you intend to override the current atom settings with those from the template default.
After setting up the menus as I intended for the entire site the intern decides to play around on the few pages they are responsible for. These no longer match the site layout as was defined in the brief, so instead of manually re-doing all the settings I can right-click and "Pull From Template Default" on each of the pages.
The option to push and pull template defaults is visible in the context menu of some atom types (accessible by right-clicking on them), and also in the Template Manager when editing a web page.
There is also a more advanced option within the Template Object itself which is covered in that Object Type's help file.
It is only possible to pull atom defaults from the template if the atom is of the same type.
If the user has changed the atom type (i.e from Menu to HTML/Rich Text) then a pull will not override the atom.