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]
Bad Example:
Multiple Conditions which should all match - such a combination of Conditions should be selected in the "ALL" list of Conditions.
See Also
The When tab
Specifying how often an Action is supposed to run (Repeat interval)
The "Keep local exec. info" option
Configuring a maximum number of executions
Selecting one or multiple conditions of which ALL must match for an Action to execute
Selecting one or multiple conditions
Combining the ALL and At least ONE list of Conditions
Restricting an Action to only run for users with a specific home/mail-server