What is a mantle in a house?

Could you please explain what a mantle actually refers to in the context of a house? I've heard this term mentioned in architectural discussions but am still unclear about its specific meaning. Is it a structural component, a decorative element, or perhaps something else entirely? Could you elaborate on its purpose, function, and any distinct features that might help me identify it? I'm quite curious to know more about this aspect of home design and construction.