What it does
I'm not aware of "Pheromone Trail" as an official Magic: The Gathering keyword or ability. It doesn't appear in any released set or in the comprehensive rules that I can verify.
It's possible this is a custom or unofficial mechanic (perhaps from a fan set, a playtest card, or a homebrew project), or the name may be slightly off from an actual ability. If you can point me to the card or source where you saw it, I can help interpret what it likely does.
For reference, real MTG keywords that involve "trails" or tracking effects don't currently exist under this name. If you meant something like a counter-based or "follow the creature" mechanic, let me know the exact card text and I'll break down the rules, common misplays, and Commander relevance for you.
Iconic cards with Pheromone Trail
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