Push notification frequency is the max number of push notifications a subscriber can receive in one day. For instance, if you set a frequency cap of 10 that means no push subscriber on that site will ever receive more than 10 push notifications in a day from your site regardless of type (i.e. manual or triggered).
If you would like to set a notification frequency cap for your subscribers, go to 'Websites' then click 'Edit' next to your website.
Next, click on the 'Misc Settings':
If you go down to 'Daily Push Frequency Cap' you can select the max number of push notifications that a subscriber can receive in one day. When you send a push its automatically skip those who have reached that limit already.
If you want to see how many were skipped, you can view this is in the campaign details. For example:
It is worth noting that the 'Sent Count' number is the total attempted to send to so don’t combine the 'Skipped (Frequency Cap)' number and the 'Sent Count' number.
Also, if you take a look at your 'Notification Logs' they will appear in the logs on the right side under 'Warning/Errors':