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 choose the bifocal/progressive lens, the ADD number should be included in your prescription.
This figure is an indication of how much extra power is required ‘on top’ of the distance prescription for near or intermediate glasses. This extra power will ALWAYS be the same for each eye and may only appear once on your prescription but it is understood to be meant for both eyes. The measurement is the Dioptre, and most values range from +0.50 to +3.50 and will go up in steps of 0.25.
The 'addition' is only required if the glasses are going to be used for reading or close work. If your glasses are for distance only, this will not be an issue. 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.
Age Add Power