When card collectors say “wax”, they’re referring to sealed packs of cards. (“Junk wax” is another common term, but the meaning is different than “wax”. See definition: What is junk wax?)
The word “wax” comes from the historical use of folded wax paper to package cards, which were then sealed with a hot roller. These wax paper-wrapped packs (“wax packs”) were one of the most common methods for packaging and selling cards from the 1930s through to 1991.
Nowadays, card manufacturers typically use plastic or foil wrap to seal cards.
To go a step further, within the card collecting hobby today, “sealed wax” usually refers to sealed boxes of cards, both retail and hobby card boxes. A collector selling sealed wax means they’re selling sealed boxes of cards.
That’s not to say that you can only refer to modern and ultra modern boxes of sealed sports cards as sealed wax. It’s still accurate and common to have card collectors refer to sealed junk wax (eg. sealed boxes of careds from the junk wax era) as sealed wax, too.
