Micro expressions are tiny facial expressions that give away what we are really thinking. They come from the non-conscious mind and are pretty hard not to show, but you can train yourself not to show them.
It turns out that successful people are able to see these tiny facial expressions in others more easily than other people. They are also able to use their own body language in better ways and can adapt their body language more easily depending on the situation they are in.
If you don’t know what micro expressions are then this TED talk by Patryk & Kasia Wezowskiis a great introduction.