/48 is netmask of ffff:ffff:ffff:0:0:0:0:0. `sipcalc` can help with this.

  $ sipcalc ::/48
  -[ipv6 : ::/48] - 0
  
  [IPV6 INFO]
  Expanded Address - 0000:0000:0000:0000:0000:0000:0000:0000
  Compressed address - ::
  Subnet prefix (masked) - 0:0:0:0:0:0:0:0/48
  Address ID (masked) - 0:0:0:0:0:0:0:0/48
  Prefix address  - ffff:ffff:ffff:0:0:0:0:0
  Prefix length  - 48
  Address type  - Reserved
  Comment   - Unspecified
  Network range  - 0000:0000:0000:0000:0000:0000:0000:0000 -
       0000:0000:0000:ffff:ffff:ffff:ffff:ffff

I remember how this works because of the IPv4 examples that I have baked into my head, e.g. 10.0.0.0/8 or 192.168.1.0/24. Clearly the first 24 bits must be 1 for that last one to make any sense.

I recently found a case where an "inverted" netmask makes sense - when you want to allow access through a firewall to a given IPv6 host (with auto-config address) regardless of the network that your provider has assigned.