Chemistry - What does dl- mean in nomenclature

That picture is showing relative stereochemistry. Essentially, it is just saying that, in the shown confirmation, the hydrogens will be on one side and the bromines will be on the other, but half of the mixture will have the bromines coming out and the other half will have the bromines going back.

If the S or R labels had been shown in the molecule (or they had used just a d- or l- prefix) the picture would have been showing absolute stereochemistry. By saying dl-stilbene dibromide, they are saying that it is a racemic mixture of the compound shown and its enantiomer, but not any diastereomers.