Template variables (TV) are simply extra content areas which can be modified on a page by page basis or use a standard default you set. Really cool stuff, also have a look at the wiki for more in depth....
Go to the manager, choose Manage Resources
go to the Template Variables tab
choose New Template Variable, give it a name and then choose what type of content.... for now choose textarea
in the default content box, add whatever you want to appear in the template (content wise)
scroll down and make sure the template is checked which you want to use this TV with
after clicking save , go to the template you just checked and edit it and add your TV call (enclose the name you entered like this: [*Name of TV*] ) into the template where you want that content to appear
from now on, any document you create that uses the template will have an extra content area which can be left with the default content you provided, or can be modified on a page by page basis. TV’s are your friend
please see the wiki and also the online docs for a better overview but this should get you going...