When you have your own IPv4/IPv6 address space, it's advantageous to announce it via your router to your ISP - especially if you have multiple providers (multi-homing). Even the lowest end EdgeRouters such as the ER-X and ERL can do a full BGP table.

The prefix lists are used to control what routes you get from your ISP, as well as the ones you send (announce).

policy {
    prefix-list BGP-ISP-From {
        rule 10 {
            action permit
            le 24
            prefix 0.0.0.0/0
        }
    }
    prefix-list BGP-ISP-To {
        rule 10 {
            action permit
            prefix 192.0.2.0/24
        }
    }
    prefix-list6 BGP-ISPv6-From {
        rule 10 {
            action permit
            le 64
            prefix 0::/0
        }
    }
    prefix-list6 BGP-ISPv6-To {
        rule 10 {
            action permit
            le 48
            prefix 2001:DB8::/32
        }
    }
}

The -From prefix lists are for routes you receive (imported) from your ISP, while the -To lists are for routes being exported (announced) to your provider. In the case of IPv4, the smallest globally accepted size most if not all providers announce is /24. For IPv6, the smallest globally accepted size is /48.

'le' means any prefix smaller (ie: 'le 48' won't allow a /64 IPv6 prefix from your ISP's routing table, but it will allow a /32). 'ge' means any prefix greater (ie: 'ge 56' won't allow a /48, but will allow a /56, /64, or even /128).