PL or Plano is Latin for "flat." If this is in the SPH section of an Rx, it means no nearsighted or farsighted correction is needed. (check out the below Rx to see how PL is used).
You can substitue "0" for Plano in a prescription as well.
"Plano Lenses" is also another way of saying "non-prescription" lenses.
How to purchase Non-prescription glasses at Zenni:
1) After you have selected your frame, you will continue to SELECT LENSES
2) Under Step 1 Prescription, you will be asked to enter your PRESCRIPTION TYPE
3) Select Non Prescription from the dropdown window, and you will not need to enter a pupillary distance (PD) or prescription.
4) You will then see our many lens options for non-prescription glasses
5) Check out and wait for your fabulous glasses to arrive at your doorstep!