What does it mean when a large piece of rock contains a thick-shelled clam fossil?
A bit hard to tell with just a single fossil, but most likely this is a bivalve of some kind that lived in the ocean.
There are some clams that live in freshwater, but most species are marine. So, its most likely the rock formation that the fossil was found in were marine sediments at some time in the past and have subsequently been lithified (turned to stone). Geologists would typically look for other evidence, including sedimentary structures (e.g. ripple marks or mud cracks), microfossils, and what is already known about the formation in which the fossil occurs.