In most applications, the primary feature of a thermal insulation material is its ability to reduce heat exchange between a surface and the environment, or between one surface and another surface. This is known as having a low value for thermal conductivity. Generally, the lower a material’s thermal conductivity, the greater its ability to insulate for a given material thickness and set of conditions.
If it is really that simple, then why are there so many different terms, such as K-value, U-value, R-value, and C-value? Here is an overview with relatively simple definitions.