It's like a pressure cooker – when the pressure builds up too much, it needs to be released to prevent it from exploding. Similarly, people need to release their stress and frustration to avoid it building up and causing problems.
Here are some examples of how "let off steam" is used:
* "After a long day at work, I like to let off steam by going for a run."
* "My brother gets really competitive, so he lets off steam by playing video games."
* "She had a stressful meeting, so she went for a walk to let off steam."
The "steam" in this idiom refers to the release of pressure, similar to the steam escaping from a boiling kettle.