SharePoint Maximum number of external user: 10,000 unique ext. users per month

Anyone, can you please help to explain more about "SharePoint Maximum number of external user: 10,000 unique ext. users per month"

Reference from http://office.microsoft.com/en-001/office365-sharepoint-online-enterprise-help/sharepoint-online-software-boundaries-and-limits-HA102694293.aspx 

It mean per month just have only 10,000 user can access site collection?

Could please help to explain more.