Respectfully I don't think they explained it well.

fc00::/7 is for "Unique Local Addresses". Basically, private, non-globally-routable addresses from which you can freely pick space. Kind of like RFC1918. It's deliberately huge and you should only use as much from it as you need. The idea being that if you merge with another organization or connect to them via VPN, it's unlikely your addresses will collide like with RFC 1918.

There's even a website (sites?) to register your ULA space on a volunteer basis to reduce collision chances.