This allows you to add a cap on the number of emails you wish to send to a recipient, for example only send 1 email every 1 days. This might be useful if you have multiple automations active but also send frequent campaigns. You may decide that automations are more important. With the contact frequency limiter set to set to only allow 1 email daily, if an automated email was sent to a recipient at 9am and you then had a campaign scheduled at 10am, the 10am campaign would not be sent as they had already reached the maximum number of emails to be sent every day.
The contact frequency limiter can be turned on in your profiles settings. Click on your profile name in the top right hand corner > Manage Profile > Scroll to the bottom > Toggle it to Limit to 1 email every X days> Enter the number of days > Save.