What does "blow off steam" mean?

Also, let off steam. Air or relieve one's pent-up feelings by loud talk or vigorous activity. For example, Joan's shouting did not mean she was angry at you; she was just blowing off steam, or After spending the day on very exacting work, Tom blew off steam by going for a long run. This metaphoric term refers to easing the pressure in a steam engine. [Early 1800s]