In the U.S., R-Value is used to describe insulation and it simply means the measurement of thermal resistance.
Thermal resistance is a measurement of temperature difference by which a material, in this case, spray foam insulation resists heat flow.
Insulation with a higher R-Value provides a more effective barrier to heat loss. Insulation with a higher R-Value leads to greater energy efficiency and cost savings, as long as the insulation is properly installed.
For example, if batt insulation isn’t perfectly cut and installed then it doesn’t meet the stated R-Value. Whereas spray foam insulation can be installed and fills every nook and cranny, making it far superior because the actual stated R-Value is achieved.
No matter what insulation is used, proper installation is a key factor.