Exploit is a keyword ability introduced in Dragons of Tarkir, where it is the signature ability of the blue-black Clan Silumgar.  When a creature with exploit enters the battlefield, the player may sacrifice a creature. Each creature with exploit has another ability that gives the player a benefit when it "exploits a creature" This means when the player sacrificed a creature because of its exploit ability.  
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 702.109. Exploit
- 702.109a Exploit is a triggered ability. “Exploit” means “When this creature enters the battlefield, you may sacrifice a creature.”
- 702.109b A creature with exploit “exploits a creature” when the controller of the exploit ability sacrifices a creature as that ability resolves.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- A keyword ability that lets you sacrifice a creature for a benefit. See rule 702.109, “Exploit.”
- You choose whether to sacrifice a creature and which creature to sacrifice as the exploit ability resolves.
- You can sacrifice the creature with exploit if it’s still on the battlefield. This will cause its other ability to trigger.
- You can’t sacrifice more than one creature to any one exploit ability.
- The extra ability doesn't trigger if the player sacrificed a creature for any other reason, including the exploit ability of a different creature.
- A player can sacrifice any creature he or she controls when the exploit ability resolves, including the creature with exploit itself.
- You don't have to sacrifice a creature if you don't want to.
- Silumgar Butcher -
Creature - Zombie Djinn
Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
When Silumgar Butcher exploits a creature, target creature gets -3/-3 until end of turn.
- ↑ Mark Rosewater. (February 29, 2016.) “Storm Scale: Khans of Tarkir Block”, magicthegathering.com, Wizards of the Coast.
- ↑ Mark Rosewater. (April 20, 2015.) “Parallel Design”, magicthegathering.com, Wizards of the Coast.
- ↑ Matt Tabak. (March 2, 2015.) “Mechanics of Dragons of Tarkir”, magicthegathering.com, Wizards of the Coast.
- ↑ Wizards of the Coast. (March 18, 2015.) “Dragons of Tarkir Release Notes”, magicthegathering.com, Wizards of the Coast.