A domain you're holding to flip is, by default, a dead asset. It sits in your account, it costs you a renewal fee every year, and it does nothing while you wait for the right buyer. Parking is the practice of making that idle name do at least a little work — either by showing ads to whoever stumbles onto it, or, more usefully, by telling the world the name is for sale.
This is a companion discussion topic for the original entry at https://namefi.io/r/en/blog/domain-parking-and-monetization