Let me go shortly,

Domain PARKING/Alias which meant is park under your primary domain and you may perform URL forwarded to your website. Example: you have 3 domain name

IamHandsome.com(Primary Domain) / IamSmart.com / IamCoolandHot.com

You may park IamSmart.com and IamCoolandHot.com under IamHandsome.com with URL forwarded. If they type any of the 3 domain and will redirect to the primary domain.