Domain parking refers to the registration of an Internet domain name without that domain being used to provide services such as e-mail or a website. This may have been done to reserve the domain name for future development, and to protect against the possibility of cybersquatting.
if u have a domain which u have no interest to develop atm u can make that domain to point some ads provider then its parked domain. if that domain gets visitors and clicks u will get some % revenue from that parked (ads provider) company