What it does
"Wraith Form" isn't a real Magic: The Gathering keyword or ability. There's no official mechanic by that name in any set or rules document.
You might be thinking of something similar—perhaps Phasing (where a permanent "phases out," becoming treated as though it doesn't exist until it phases back in), or cards involving Wraiths as a creature type (like the Nazgûl in the Lord of the Rings set, which have the Ring-bearer mechanics). It's also possible you're recalling a custom or homebrew ability, or mixing up terminology from another game.
If you can tell me which specific card or effect you encountered, I'm happy to explain the actual mechanic and its Commander relevance—including the rules nuances people commonly misplay. I'd rather give you accurate information than invent rules for a keyword that doesn't exist.
Iconic cards with Wraith Form
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