The number printed at the bottom of a card is usually just bookkeeping — until it goes higher than the set total, which is when things get interesting.
What the numbering tells you
A set with a printed total of, say, 84 cards will have its main numbered cards run from 1 to 84. Anything numbered higher than that total is a Secret Rare — a bonus card added on top of the main set.
Not one single tier
Secret Rares aren't a single rarity level; they span several tiers of their own, typically running from Illustration Rares through Special Illustration Rares up to Hyper Rares, each scarcer and more elaborate than the last.
Why they dominate set value
Secret Rares combine the lowest pull rates in the set with the most elaborate art treatments going, which is exactly the combination that tends to produce a set's most expensive and sought-after cards.
Frequently asked questions
What makes a card a Secret Rare?
A collector number higher than the set's printed total.
Are all Secret Rares the same rarity tier?
No, they span several tiers from Illustration Rare to Hyper Rare.
Why do Secret Rares often become a set's most valuable cards?
Low pull rates combined with elaborate art treatments.