What does the numbers 39 40 and 41 after the element name potassium refer to?

The numbers 39, 40, and 41 after the element name potassium refer to the mass number of the isotopes of potassium. Potassium has three isotopes: potassium-39, potassium-40, and potassium-41, which have 19, 20, and 21 neutrons respectively.