"Circumstances" means a condition that affects an action.
Example #1: "I wanted to skip school, but the circumstances didn't allow me to."
Sentence #1 is saying that the speaker wanted to skip a day of school, but he couldn’t because of specific conditions that affect that action. These conditions could be that he would get in trouble or his parents wouldn't allow him to.
Example #2: "I would travel the world, but my current circumstances won't let me."
Example #2 is stating that the speaker wants to travel the world, but specific conditions won't allow him to commit that action. These conditions could be him having no money or time.