It means they had recently consumed marijuana enough to be consider under the influence of it. Marijuana that had not yet cleared the bloodstream. Active comes from active metabolites, their body was still metabolizing recently consumed marijuana.

That's in opposition to the inactive metabolites which are stored in fat and slowly secreted via urine for a period much longer than someone is under the influence of marijuana.

So saying active means that their body was still processing it and the brain CB receptors are still being engaged. Whether or not they were "high" might be debatable, but considering it's showing up in a FAR higher amount of the people involved in fatal accidents than any group of the general public it's a pretty reasonable conclusion that it played a role.