The process or fact of sharing in an action, sentiment, involvement in a matter or event, especially one in which the outcome directly affects those taking part. Oxford English Dictionary.

Many persons are shut out from any participation in political power. J. Bright, Speeches

It’s your activity in an artistic endeavour such as a play or film. Or when you make that crucial collegial agreement to throw the ball back and forth with your scene partner. It’s the opposite of being a bystander. If you’re participating you can take it easy.