Someone out there hates the character and wants him dead. The value of the Hindrance depends on how powerful the enemy is and how often he might show up. A Minor Enemy might be a lone gunslinger out for vengeance. A Major Enemy might be a supernatural gunslinger who wants your hero dead.
If the enemy is one day defeated, the GM should gradually work in a replacement, or the hero may buy off the Hindrance by sacrificing an Advance.
Savage Worlds Deluxe Edition p. 31