The ADD—short for Reading Addition—is the additional correction required for reading. It can be used to make either reading glasses, bifocal glasses or multifocal glasses. If you wish to choose bifocal/progressive lenses, the ADD number should be included in your prescription.

The "addition" is only required if the glasses are going to be used for reading or close work. Please kindly note that if you have ADD and choose distance glasses, the glasses can only be used for seeing far, not available for reading. Sometimes opticians use the word "Add" or "Near" instead of "addition". They may only write it once, but it normally applies to both eyes and is almost always the same value for both eyes (e.g. 'Add' +2.50 - should be entered for both eyes).

Here is a reference：