Activation makes sure that the AUT is in focus at the beginning of test execution. This is acheived by clicking somewhere in the AUT window. You can specify the activation method (i.e. where to click) as part of a configuration for an AUT , or you can create a Test Step within a test to do the same thing -->Reference Manual .
The advantage of specifying an activation method here is that it is central and affects each test execution started on this AUT with this configuration.
Bear in mind that you may need to activate your AUT in order for tests to work, especially if the AUT runs on the same machine as Jubula .