What it does
I'm not able to explain "Matter Absorption" because it isn't a real Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or on any official cards, so I can't describe what it does without inventing mechanics—which would only mislead you.
It's possible you're thinking of a similarly named ability, a card from a custom/un-official set, or a term from another game entirely. Some real keywords that involve "absorbing" or converting things include Absorb (prevents a set amount of damage to a creature) and Exploit or Devour (sacrificing creatures for value).
If you can share the exact card name or where you saw "Matter Absorption," I'll gladly explain how it actually works and how it matters in Commander.
Iconic cards with Matter Absorption
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