Inactive wikis are those with few articles and very sporadic edits. They have no community and no users dedicated to editing them. These occasionally accumulate Wiki Spam and vandalism and waste the valuable time of those users who check these small wikis for such edits but obviously do not really want to do the job in the first place as is indicated by their support of such proposals as this.
Another proposal can be found at SWMT, for those who like monitoring inactive Wikipedias, and vandalism on these wikis can be reported at vandalism reports.
It is possible for a developer to lock an inactive wiki to prevent it from being edited whilst not removing any existing content. Proposals to close projects can be created at Proposals for closing projects.