The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
The package declares a build-depends on an essential package, e.g. dpkg, without using a versioned depends. Packages do not need to build-depend on essential packages; essential means that they will always be present. The only reason to list an explicit dependency on an essential package is if you need a particular version of that package, in which case the version should be given in the dependency.
Refer to Debian Policy Manual section 4.2 for details.