When you park a domain with someone, it means that they have DNS records for that domain. If you have a domain parked, you can be sure that no one can register it for you. Parking is free with most hosting providers (including us). You can then transfer the parking to the hosting at any time or have the parked domain directed to a hosting you have somewhere else, such as a competitor, for free.
Domain parking is usually used to seize a domain with which the user has plans for the future, to protect the user against domain speculators, or the user parks just because he is a domain speculator.
Domain parking should be used for similar domains to the main domain that will redirect to the main domain.