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.