In subversion you can set properties for versioned files and folders. There are some pre-defined properties that start e.g. with "svn:". To read and write properties in TortoiseSVN, you can use the context menu. Right click on some file or folder and then go to "TortoiseSVN → Properties".
In the upcoming diolog you can see and modify the properties that exist already. With "New..." you can set new properties. Some properties can then be selected and configured directly. To obtain all properties choose "Other".
A new dialog appears. Here you can select the "Property name". In "Property value" you can set the needed value.
You can set the properties either for a folder or for one or more files. If you set it for a folder you can set the properties optionally to all subfolders and containing files. To do that check "Apply property recursively".