FreeLists' ListRank is a means of determining the relative quality of a mailing list. It is by no means an endorsement of a list, and FreeLists by no means guarantees that the lists with high ListRanks are better than those with low ListRanks and vice-versa. It's mostly a convenient way of ordering lists in search results and category listings with the added benefit of putting mailing lists that are obviously useless (abandoned, never used, etc.) at the bottom of those results.
Several criteria are considered when building a list's ListRank, such as the percentage of subscribers who participate in discussions, the age of the list, etc. To prevent abuse of ListRank, however, that list of criteria must be kept secret.