Inside a claim, players outside the guild that holds it cannot build or break. That is the protection, and it applies from the moment the cell is claimed rather than after an admin approves anything.
A player with no guild still gets a personal claim allowance, which matters more than it sounds: on most survival servers the answer to griefing is "join a group or lose your house", and here it is not. Founding a guild raises the ceiling rather than unlocking the floor.
Land only changes hands through a declared war, and even then vault chunks are exempt at every guild level. A server can also close wars entirely for stretches of the calendar through its Peaceful Era setting, which is the closest thing to a build-only mode the system has.
The exact numbers behind all of this, claim capacity per guild level, the war clock, tribute rates and the sovereignty bonus, are in the guilds and kingdoms guide.