Site authorisation is an optional feature that allows you to identify only specific sites that are permitted to use your Google ad code. If you're particularly concerned about malicious use of your ad code by others, you may wish to use this feature as a precaution.

Site authorisation is linked to Site management: sites that you add to your list of authorised sites are also added to your Owned sites list. Learn more about managing your sites.
Authorizing your sites

    Sign in to your account.
    On the Home tab, visit the Account settings page.
    In the "Access and authorisation" section, next to "Site management", click edit.
    Select the box marked Only allow certain sites to display ads for my account.
    Enter the URL of the sites that you want to authorise, then click Save.

Your changes should take effect within 48 hours.
What happens to sites that I don't authorise?

If a URL displaying your ad code is not on your list of authorised sites, ads will still show on that URL and impressions and clicks will be recorded. However, advertisers will not be charged, and you will not receive any earnings for that site.

Our systems might attribute a few impressions to sites that you do not control and that do not directly have your ad code on them. Check possible reasons for this.

Note: If you don't add a site to the authorised sites list, and you place your ad code on it, ads will continue to appear on the site but you won't earn from them. Therefore, please enable this feature with caution to avoid missing out on valid revenue.

Source: https://support.google.com/adsense/answer/65062