What is a reserved domain name?
Reserved domain names are domain names that the central registry has held back from general registration. This means that they are not available to register.
The reason for this can vary. In some cases, certain domains may be held back due to potential "name collisions." Often, registries will decide to not release one and two character domain names. And sometimes registries will hold back certain domain names they consider valuable. For example, the .ME registry has held back certain premium domain names that you can only register by application.
Please note that sometimes registries decide to release previously reserved domain names. Generally when this is done, the reserved domain names are considered premium by the registry and sold at a higher price.