When something is monolithic it's big, and made of one thing. A large piece of stone jutting from the earth is a monolith, and Detroit's economy when it depended entirely on the auto industry was monolithic.
Broken into its roots mono and lithic, monolithic means simply "one stone." When monolithic is used to describe something societal — like a religion or an organization — it has a slightly negative connotation. For example, a monolithic society is rigid and homogenous, not open to new ideas. Being monolithic is good for rocks, but not so good for groups of people.