A microexpression is that fleeting, involuntary facial expression that appears on people's faces when they experience emotions. They often happen as fast as 1/15 to 1/25 of a second. It's one of the most difficult things to master as an actor, but if you can manage to do it you'll be able to pull off a sterling performance.
There are seven universally recognized microexpressions. These are:
Disgust microexpressions include a raised upper eyelid, lower lip raised, nose wrinkled, cheeks are raised, wrinkles show below the lower eyelid. Think about smelling something bad to get this look correct.
Anger microexpressions include eyebrows are lowered and drawn together, vertical lines between eyebrows, lower lid is tensed, eyes are staring or bulging, lips pressed firmly together, nostrils dilated, lower jaw juts out.
Fear microexpressions include forehead wrinkles in the center between the brows, eyebrows raised and drawn together, upper eyelid is raised but lower is tense and drawn up, whites of eyes shown above iris but not below, open mouth with tense lips.
Sorrow microexpressions include inner corners of the eyebrows are drawn in and then up, skin below eyebrows looks like a triangle with inner corner up, corner of lips are drawn and down, jaw comes out, lower lips pouts.
Joy microexpressions include corners of lips are drawn back and up, mouth may or may not be parted with teeth exposed, a wrinkle from outer nose to upper lip, cheeks are raised, lower eyelid may show wrinkles, crow's feet near outside of eyes.
Surprise microexpressions include raised and curved eyebrows, horizontal wrinkles across the forehead, eyes are wide open showing the white above and below the iris, jaw drops open.
Contempt microexpressions include one side of mouth raised.
Microexpressions are most often used by actors when other characters are speaking on stage. Your character is reacting to what they're hearing from other characters; this is where micro expressions play an important part in your performance. Ignore them, and you'll appear flat and static on stage. Use them, and you'll have the audience understanding and following your every move.
Here's a great online article that includes example photos of each microexpression: https://www.scienceofpeople.com/microexpressions/.