The Pleiades is the brightest open cluster of stars we can see with the unaided eye in the northern sky. A nebula - a cloud of gas and dust - is passing through the cluster and the dust is affected by the Pleiades stars. You can see how the dust around the blue reflection nebula is filamentary and stratified. And by the way, if you look in the lower left of the image, you should be able to spot Comet C/2015 ER61 (PanSTARRS).
Image Credit & Copyright: Juan Carlos Casado (TWAN, Earth & Stars), Miquel Serra-Ricart & Daniel Padron, FECYT