"She decided to leave her job" is a sentence talking about a single action that happened in the past and that's all (past simple tense). It comes with a specific time, although it may not be implicitly stated.
"She had decided to leave her job" is a sentence talking about one action in the past which is followed by another action in the past (past perfect tense), although the other action doesn't need to be implicitly stated. It will be part of the context.
Person A: "What did you find when you got home?".
Person B: "(I found that) the windows had been broken".