A branch or tag is simply a copy of the trunk, or in some cases another folder, in your Subversion repository. To create one, use the “Browse” view to navigate to the right place in your repository and drag the trunk of your project to the “branches” folder while holding down the “Option” key on your keyboard. When you drop the trunk, a dialog will open asking you to name your new tag or branch.
For more information about tags and branches, see: About “trunk”, “tags” & “branches”.