Simple is a role modifier that modifies the way in which an active action is used; it causes the action to fail, as though it were roleblocked, when used against a player who is not vanilla. In other words, only Vanilla Townies and Goons of the various anti-town factions will be affected by a Simple action; other players are too complex to be vulnerable.
Simple is an acceptable modifier under Normal guidelines on mafiascum.net, when used on an active role belonging to a player of any Town or Mafia/Werewolf alignment. It causes the role to behave as though it were roleblocked when targeting a player who is not vanilla.
Use and Power
Simple roles have somewhat different uses in practice depending on the role.
With an investigative role, making the role Simple effectively adds (unreliable, but still fairly reliable) Vanilla Cop functionality via the No Result result. If the original role could differentiate a Vanilla Townie from a Mafia Goon, there are now effectively three results, leading to a slightly stronger Neapolitan.
With manipulative roles, making the role Simple effectively prevents them being used on anything other than a factional kill. This is normally beneficial to a Town user of the role, as it prevents them affecting other townies' roles by mistake (a Vanilla Townie would have no actions to affect), although not as beneficial as being Disloyal would be (as it won't affect Mafia power roles even if they're "spending a night as a Goon" rather than using their role).
With protective roles (assuming Town-sided), Simplifying them removes one of their main functions (that of keeping other power roles alive and thus gaining more uses of their ability), thus restricting them to only being able to save strong Town voices. A Simple Doctor has no useful interactions with other Town roles, and thus can be substantially weaker than an unmodified Doctor (but will be almost as strong in a setup where there were no useful interactions anyway). This is one possible method of breaking up Follow the Cop (via modifying the Doctor; it's thus the converse of Macho, a modifier that achieves the same thing when placed on the Cop).
The opposite of this modifier is Complex.