A feeling that can cause psychological effects. The precise definition is debated. There are ways around it, like stoicism, acceptance, and asceticism. Some amount is needed, sometimes, but other times not so much, like stress! It's a bit tricky, because its existence affects how objective critical thinking may be. Still, it needed to exist as an alternative to deep thinking, due to you not being able to have time of whether to run away from a predator: Darwinism strikes again!
Emotional expression is showing how you feel to the outside. It is
fine. Lack of it is also fine. You could live however you want. But society may require
it, rarely, for instance, in customer service jobs (a social mask
occurs here).
Expressing negative emotions could be risky though, depending on the
setting. Be just a bit careful, unless you are with yourself, or know
you are safe, e.t.c.