This is a string property for your own use. It is usually used to "label" what its parent ScriptLine does. For example, in an Insert, the Label may be "Notes Page Every Month" or "Re-Order Form at End of August".
Depending on the ScriptLine, the Label text may also appear in the Scripts Editor.
Although you can enter text up to a length of 254 characters, if the text you enter cannot be fully seen in the Property Inspector, it is better to use the Comments property. Generally you should set a Label for Insert ScriptLines, since they usually need some explaining. Otherwise it is usually best to set Labels only when something out of the ordinary is going on (remember the boy who cried "wolf").