The wiki ID (or database ID) is the most common way of identifying a wiki within a wiki farm in MediaWiki.
When every wiki is in a separate database, it is simply the database name; when multiple wikis share a database and are differentiated with table prefixes, it is in the form
<db name>-<table prefix>.
It is used for getting a database connection for a given wiki (see
LoadBalancer::getConnection()) and in $wgConf, among other things.