Hello Giovanni
The General Prefix is a feature that is used to easily migrate a router’s IPv6 addresses from one general prefix to another. If you’re not sure what a general prefix is, then that sentence probably sounds cryptic. Take a look at this lesson to find out more about it and it should make sense.
Now in this particular case, the general prefix feature is being used on an interface that is performing 6to4 tunneling. Specifically, what the command does is (according to Cisco):
Defines a general prefix for an IPv6 address.
When defining a general prefix based on a 6to4 interface, specify the 6to4 keyword and the interface-type interface-numberarguments.
When defining a general prefix based on an interface used for 6to4 tunneling, the general prefix will be of the form 2001:a.b.c.d::/48, where “a.b.c.d” is the IPv4 address of the interface referenced.
You can find out more about how this feature is implemented at the following Cisco documentation, which is where the above quote was taken from…
I hope this has been helpful!
Laz