> - Explicit dependency injection. I've never understood what this means
The article seems to mean what's described in this other article as "dependency parameterization" where the dependency is explicitly passed to the function (and every function it calls that also needs that same dependency). This is as opposed to, in OO languages, setting the dependency (typically) during construction (however the object is constructed). Or it's otherwise set in some larger scope than the functions which make use of it (global, module, object instance, whatever is appropriate to the language and task).