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.
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
Combining the ALL and At least ONE list of Conditions
Selecting one or multiple conditions out of which At least ONE must match for an Action to execute
Restricting an Action to only run for users with a specific home/mail-server