The firmness of a mattress refers to its resistance to pressure exerted on its surface. More specifically, it measures the mattress's ability to support the sleeper's weight evenly, without creating excessive pressure points or depressions that could lead to pain or discomfort.

Mattress firmness is typically measured on a scale of 1 to 10, where 1 represents extreme softness and 10 represents extreme firmness. Most mattresses fall somewhere in between these two extremes.

It’s important to note that the perceived firmness of a mattress can vary from person to person based on a variety of factors, such as weight, height, preferred sleeping position, and personal comfort preferences. For example, a heavier person will tend to prefer a firmer mattress for adequate support, while a lighter person might feel more comfortable on a softer mattress.

In general, a firmer mattress is better for back and stomach sleepers because it helps maintain good sleeping posture by supporting the spine in a neutral position. On the other hand, side sleepers often prefer a softer mattress that allows their shoulders and hips to sink slightly into the mattress for better spinal alignment.

In summary, mattress firmness is an important factor to consider when purchasing a mattress, as it can have a significant impact on the sleeper's comfort and support during the night. It is advisable to test different firmness levels in store to find the one that best suits your personal sleeping preferences.