A firm mattress refers to a type of bedding that provides minimal give or sinkage, offering good support and alignment for the spine while sleeping. It is typically made with materials such as memory foam, latex, or innerspring coils, which are designed to provide a stable surface that doesn't compress easily under weight. A firm mattress can be beneficial for those who prefer a more rigid feel and need extra support to help alleviate pressure points or back pain. However, it may not be as comfortable or conforming to the body shape as softer options, and some individuals might find it too hard or unyielding over time.