Checking the "ALL" checkbox allows to select one or multiple Conditions, ALL of which must match for an Action to effectively execute.
Good Example:
[Client has 2 GB of memory] and [Client has 2 GB of free disk space]
Bad Example:
[Client is laptop] and [Client is desktop]
A client cannot be both of type laptop and desktop - such a combination of Conditions should be selected in the "At least ONE" list of Conditions.