The Stop processing more actions action stops the action sequence.
Any action after this action will not run.
Although this is useful in a normal action set, this is also helpful when trying to debug or test a long action sequence.
Example: You have an action sequence which applies a percentage to a balance amount for a customer, and then sends it out using a custom email template.
You want to change that calculation, and thus you need test a new percentage calculation. To make sure it doesn't send out the email, you can use Stop processing more actions to halt the action sequence after it shows you what the calculation is (probably in a Show Message action).