A slab is a great big thick piece of something. Your backyard patio, for example, might be made of a huge slab of concrete.
A sidewalk is often composed of one cement slab after another — these are sometimes called paving slabs. You could refer to large pieces of stone or wood as slabs too. Even your lunch might be a slab, if it's a thick hunk of cheesy pizza or a huge slab of bread slathered with peanut butter. The origins of the word slab are unknown, though we do know it's been around since the thirteenth century.