When you 'let off steam' or 'blow off steam' it means to do something in order to help manage strong feelings emotions or a large amount of energy. Usually, when you 'let off steam' or 'blow off steam' you are doing something constructive rather than behaving in a way that expresses your anger or frustration or another negative feeling. This is a casual expression usually used in social contexts.

1. I like going for a run in the evening. It allows me to let off some steam after a tough day at work.
2. I know things are difficult in your relationship at the moment, so feel free to give me a call if you ever feel like letting off some steam. 

1. The performance review gave me a chance to blow off some steam about how badly I felt the project was organized and managed.
2. We have an on-site gym and relaxation room where employees can let off steam after a difficult day.