What it does
"Trance" isn't an official Magic: The Gathering keyword or ability. There's no card or rules text in the game that uses "Trance" as a defined mechanic, so I can't accurately explain how it works, its rules nuances, or its Commander applications without inventing rules.
It's possible you're thinking of a similarly named ability or a term from another game. Some real mechanics that might be confused with it include "Threshold" (abilities that turn on once seven or more cards are in your graveyard), "Delirium" (cares about four or more card types in your graveyard), or possibly a custom/homebrew keyword from a non-official set.
If you can point me to the specific card or set you saw "Trance" on, I'll gladly explain the actual mechanic it uses and how it plays in Commander.

