What is Lockdown mode?

Just a quick question from someone still learning all this Joomla jargon. I was trying to figure out a way to allow a certain number of people to register on our ES site to start testing it, but then shut down registrations while it remains viewable by the public. Is that what Lockdown mode is for? And if so, what is the purpose of the option to allow registrations in lockdown mode? Doesn't that defeat the purpose, or why exactly is that option available?

Of course, if lockdown mode doesn't allow for the purpose we are looking for, what other option might there be to do so?