Checking the "At least ONE" checkbox allows to select one or multiple Conditions out of which at least one must match for an Action to effectively execute.
Good Example 1:
[Client is laptop] or [Client is desktop]
Good Example 2:
[Client operating system is Windows] or [Client operating system is Mac]
Good Example 3:
[Client is in IP segement A] or [Client is in IP segment B]
Multiple Conditions which should all match - such a combination of Conditions should be selected in the "ALL" list of Conditions.