The Sunrise and Landrush periods come before the time when a domain extension is open to the general public for registration, although Landrush is not mandatory.
This is the initial period in which trademark owners get first dibs on domains that are same or similar to their trademark. The Sunrise lasts for 30 days or more, and the registry must provide a public notice 30 days prior to the Sunrise start date. The purpose of the Sunrise period is to prevent cybersquatting of trademarked domain names.
This is the period after the Sunrise, in which, premium or generic domains get sold to interested parties at a higher price. The Landrush period can be bypassed altogether by the registry and can go straight to general availability, but this usually does not happen. The Landrush period gives the registry the opportunity to make bigger profits on some of their best domains. During this time frame, the registry may decide to sell the domains via auction or on a first-come, first-serve basis.
General Availability Period
This is the period after Landrush, in which any qualified person in the general public may register a domain.
So that explains what happens just before a gTLD is introduced to the general public.
Want to know what process a domain name goes through once it expires? Click here.