This is a transition mechanism in which the user configures a 6to4 client in their PC or home gateway. The 6to4 client receives dynamic tunnel services from a 6to4 server which is found via the anycast address prefix 192.88.99.0/24 allocated in RFC 3068. This tunnel then attaches the IPv4 host to the IPv6 network using the IPv6 address 2002:V4ADDR::/48. The mechanism is documented in RFC 3056. Most 6to4 implementations allow a relay router to be configured as an alternative to the RFC 3068 well-known relay router address. That address is exactly 192.88.99.1.
A complete explanation about 6to4 is available at The IPv6 Portal.
ISPs can improve connectivity for their customers who are currently running IPv6 on their PCs by setting up a 6to4 relay. This avoids the increased network latency caused by a trombone path to the IPv6 destination through a distant 6to4 relay. For an ISP's customers to find that relay it either needs to be explicitly configured in their client stacks, or it needs to be numbered 192.88.99.1 and the clients need to use the RFC 3068 address.
In addition, a content provider can also add IPv6 access to their services by configuring 6to4 on their network. Again, by shortening the path taken by one of the protocols, you ensure that there is no tromboning of the path and network latency is close to the minimum possible. Of course, you need to configure all the servers and related infrastructure responsible for those services to use IPv6, using a 6to4 prefix. Note that this is not particularly different from any other kind of IPv6 transit a content provider might decide to arrange.
For more info on specifics using Cisco, Linux and BSD, have a look at the First Steps for ISPs page.