We are talking about sleeping mats, whether inflatable, foam or self-inflating, and insulation. R-value is a rating measuring a mat's thermal insulation against the ground. The higher the R-value, the better the insulation. Note that a well-insulated mat does not warm you up but reduces heat loss from your body to the ground.
I explain how it is measured and how to use it.
To begin, here is a small diagram summarising how heat can be lost to the ground: