This means the goggle frame is designed to accept a prescription lens insert, but you'll need to purchase that insert as an add-on accessory. The insert is a small frame that clips inside the goggle behind the main lens, and you take it to your optometrist to have your prescription filled.
This option is great if you want flexibility—you can start with the goggles and add the Rx insert later if contacts or an OTG (over-the-glasses) setup aren't working out. Once installed, it delivers the same fog-free, clear vision as goggles that come with an insert included.
Just factor in the extra cost: the insert itself plus your prescription lenses can add $50–$150 on top of the goggle price. If you already know you need vision correction on the slopes, compare the total cost against goggles that include the insert or offer direct Rx lenses—it might be more economical to go with one of those options instead.