Traffic source is a previously created set of tokens which you can use in your campaigns. Once you've created traffic source, when creating campaign in the tab "General info" you don't need to fill in tokens manually. But, you will not be able do edit the tokens in this case. Just select traffic source from the drop-down list. If you would like to add tokens while creating or editing the campaign, you may choose Traffic sources->Custom menu.
To create traffic source, go to the Traffic source section on the left-side panel. Here you may use a list of pre-defined templates of traffic sources - just click the Traffic source templates button and choose the needed one. You can also add Tokens, Tracking id and Cost parameter manually if needed. After creating your traffic source, click on Save button. You have the abillity to edit your traffic sources in the Manage traffic sources menu.