What it does
"Hire a Mercenary" isn't an official Magic keyword or ability—it doesn't appear on any card or in the comprehensive rules. You may be thinking of something else, so let me point you toward likely candidates.
If you mean Mercenary creatures, that's a creature type from the Mercadian Masques block. Many had an activated ability letting you tap them to search your library for a creature with mana value equal to or less than theirs and put it onto the battlefield tapped—effectively "recruiting" reinforcements. In Commander, a Mercenary engine can chain out a whole board, but the fetched creatures enter tapped and can't attack or block immediately.
If you actually meant a specific card or mechanic, double-check the exact wording. Tell me the card name and I'll explain precisely how it works and where it shines in EDH.
Iconic cards with Hire a Mercenary
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