Comfort refers to the feel of the top of the mattress when pressed against your body surface. In other words, it is an amount of softness you feel or the level to what your body sinks into the mattress. Comfort is provided by adding upper padding layers, sometimes also called upholstery layers, on the top the support system. It is primarily a matter of personal preference.
There are myriad options available and it takes time to choose ideal type and level of comfort. Typically, model descriptions like firm, plush and pillow top refer to the amount of padding, and thus the overall comfort level and softness of the mattress. With the firm model having the least amount and pillow top having the most amount of padding on top.
This is where you will be spending your precious time laying on the mattresses trying to find the perfect fit. Please be advice, do not rush this portion of the process. More time you spend laying on mattresses in the showroom better is your chance to pick the right one first time. Some people can decide rather quickly, where others need hours to make that decision. If you have your doubts, and even after some time you are undecided, do not force it. Come back one more time for a quick visit to make the final decision.
Some manufacturers will provide ILD number that correspondents to mattress firmness. ILD stands for Indentation Load Deflection and refers to mattress firmness. ILDs of 14-22 are often considered soft; 23-29 medium; 30-37 firm; and 38 and higher extra firm. Most people prefer an ILD of 25-35.