A glasses prescription can look like a wall of abbreviations and plus-or-minus numbers. It is really just a short table describing what each eye needs. Once you know what each column stands for, your own prescription becomes easy to read.
OD and OS — your two eyes
Prescriptions use Latin abbreviations for each eye. OD (oculus dexter) is your right eye and OS (oculus sinister) is your left. Some prescriptions use plain 'Right' and 'Left', and you may see OU (oculus uterque), meaning both eyes together. Each eye gets its own row of numbers.
SPH — the main correction
SPH, or sphere, is the main lens power, measured in dioptres. A minus value means you are near-sighted (distance is blurry); a plus value means you are far-sighted. The further the number is from zero, the stronger the correction. A value like -2.00 is a moderate near-sighted prescription.
CYL and AXIS — astigmatism
These two work as a pair and only appear if you have astigmatism, where the eye is shaped slightly more like a rugby ball than a football. CYL (cylinder) is the amount of that correction, and AXIS is the angle it sits at, written as a number from 1 to 180. If one is filled in, the other will be too; if both are blank, you simply do not need this correction.
ADD — reading power
ADD is extra magnifying power added to the lower part of the lens for close-up work. It appears on prescriptions for bifocal or progressive lenses and is always a plus value, often the same for both eyes. It is what makes reading comfortable when your eyes no longer focus up close on their own.
PD — pupillary distance
PD, the pupillary distance, is the gap between the centres of your pupils in millimetres. It tells the lab where to place the optical centre of each lens so the correction lines up with your eyes. It is essential for accurate glasses, and is sometimes measured separately from the main prescription.
A note on using it
A prescription describes your eyes at the moment of the exam and usually carries an expiry, since vision changes over time. It is also specific to glasses — a contact-lens prescription is measured differently and is not interchangeable. When in doubt about any figure, ask the professional who wrote it; that is exactly what the exam is for.