In more detail
RFC 4193 splits fc00::/7 in two: fd00::/8 holds locally assigned addresses with a pseudo-random 40-bit Global ID, while fc00::/8 was set aside for a centrally assigned scheme that was never defined. In practice a ULA is fd followed by the random 40-bit global ID, making a /48 prefix such as fd12:3456:789a::/48; the randomness makes clashes between networks unlikely. The next 16 bits number subnets, giving 65,536 /64 networks per site. It is the IPv6 counterpart of 10.0.0.0/8 and 192.168.0.0/16.
Why it matters
Picking an easy prefix such as fd00::/48 instead of a random one defeats the purpose: when two networks that chose the same prefix are later joined by a VPN or a merger, their addresses collide. ULAs are not reachable on the internet, so devices that need internet access also keep a global address alongside.
Example
In fd12:3456:789a::/48 the Global ID is 12:3456:789a (40 bits). Subnet 0001 gives the network fd12:3456:789a:1::/64, and a host in it might be fd12:3456:789a:1::10. Generate a random prefix with the IPv6 ULA Generator.
Common confusion
Link-local addresses (fe80::/10) work only on a single link and exist on every interface automatically; ULAs are routable across a whole organisation.