Tags are a way of identifying objects within your GUI by name. They can be used the same way as joins.
To assign a tag to an object, you must first show the tag editor via guiDesigner main menu > View > Tag Editor. You can then select any object on a page, then focus on the tag editor to begin assigning a tag. The GUI object you have selected (button, slider, etc) will begin flashing green to show that it is the target of the tag assignment.
Objects can have multiple tags, separated by spaces. The same tag can also be assigned to multiple objects. This allows you to group objects that you want to manipulate.