Google has a system that is prepared to examine each click on the Adwords Ad. Clicks on ads that Google considers to be illegitimate, such as unintentional clicks or clicks resulting from malicious software. When Google identifies that the clicks are invalid, they will try to automatically filter them from the reports and the payments so that you're not charged for those clicks. If they find that invalid clicks have escaped automatic detection, you may be eligible to receive a credit for those clicks. These credits are called "invalid activity" adjustments. They don’t give you refund in terms of money.