The actions targets allows you to choose who the actions will be applied on. As for the conditions, this step will change according to the trigger you've chosen.
If you chose a classic trigger then you can execute actions on all the users or just a part, depending on the filters you've chosen. If you've chosen a trigger based on a user action you'll be able to execute actions for all the users and just the one who triggered the automation.
Note that if you want to execute your actions on all your users, save directly without setting a filter. This way AcyMailing will execute the automation on all users.
If you click on the See users button a modal will open with all of the users matching the current condition.