It is essentially how long your foot is in centimeters.
Use the ruler to measure the length of your foot from the longest toe to the back of your heel.
Knowing that a bigger size of slipper can make a world of difference when it comes to finding comfortability

SIZE	EUROPE	FOOT LENGTH CM
XS	34-35	23,5
S	36-37	24,5
M	38-39	26
L	40-41	27,5
XL	42-43	28,5
XXL	44-45	30