The rock itself is bioturbated siltstone - that is, fine-grained sedimentary rock that has evidence of animals living in it. Such fossilized traces of life are called trace fossils (ichnofossils). Trace fossils include dinosaur footprints, but also much more mundane things, such as worm and seashell burrows. Your specimen contains burrows, often appearing as round or oval or distorted dots - depending on how they intersect the surface. The large dark curved mark may also be part of a burrow, or it could be an external mould of a fossil (the original fossil no longer being present), or it might be nothing of significance at all - difficult to be sure from the photo, given that some erosion has also occurred.