Locking a Topic means there is a way to unlock it. When checking the box under the 'Lock this Topic?' tab, you will be asked which other Topic in the Project the player has to complete in order to unlock this Topic.
Basically, locking a Topic means that the player will have ways to unlock it, and access it.
Making a Topic private means it will be inaccessible to players until the game admin has invited them to the Topic.