A "minor antagonist" is an antagonist role which can exist in any roundtype, either by admin spawn or as a ghost role.
The main distinction between standard and minor antagonists is level of severity, usually a minor antag will be getting up to no good, but not. Your exact concept will depend on the type you've rolled.
A thug is usually a disgruntled person or crewmember, cleared to break the law and crack skulls. Something or somebody really pissed them off, and now they're going to break something.
A renegade is the "good guy with a gun" archetype, for better or worse. Their primary goal is to defend themselves, usually using an unlicensed weapon they've smuggled aboard. If somebody crosses you, they're going to pay.
Note that a renegade is not an official member of security and will probably run into trouble with the law for their vigilante actions.
Ghost roles are selected from eligible observers in-round. This includes stowaways, cursed swords, and magic corgis. They should follow their roleplay guidelines found in the flavour text.
A stowaway is a ghost role spawned in mid-around. This can be 'just' a stowaway, or a combination with Thug or Renegade.
You will spawn in maintenance as a trespasser, and are not a member of the crew. How the crew reacts to this is likely to vary greatly, so be cautious.