Tags are applied to rhythms to make them easy to find and group.
In the following example you can see the save rhythm window. This is shown when:
When you save a rhythm you can give it a name and choose any tags (a) you want to apply to it. You can also add new tags yourself to apply to it by clicking the Master Tags List button (b).
The Master Tags List window is shown next, you can use it to change the name of an existing tag or add/delete tags. You'll also find this window under the 'More...' menu so you can modify tags without saving a rhythm.
Note: You can change the name of a tag (right-click for popup menu options). You might prefer your one name, or perhaps it's in a different language to your own. If you later re-import the rhythm that it came with, your new name will be preserved.