This is advice often give to aspiring novelists. The idea is that your story will be more vivid if you let the characters of the people you are writing about emerge from their actions than if you just describe what they are like. (There’s a much fuller explanation on Wikipedia.) ‘Actions speak louder than words’ expresses much the same idea.
When you write your witness statement, you are telling a story. Unlike a novel, your statement must be true. But ‘show, don’t tell’ is still good advice. Don’t say “Miss Claverham behaved disgracefully by doing so-and-so… ” or “It showed how Miss Claverham always wanted to put me down when she said… ” Just say what she did, and what she said. The tribunal will have to decide for itself what it thinks her behaviour demonstrated, and whether it was disgraceful – or, more to the point, unlawful. Your account will have more impact if you resist the temptation to load it down with judgment and comment of your own.
As with most rules, there will be exceptions. If you are claiming compensation for injury to feelings in a discrimination case, you will have to say how the discrimination has made you feel. Sometimes it is just too time-consuming to show: if, for example, a judgment about someone’s character is part of the background to you story, but not a central part of what you have to prove in order to succeed in your claim, you may want to say “Everyone found Miss Claverham difficult and demanding, and none of the secretaries wanted to work for her” instead of taking several pages to describe sufficient instances of her behaviour to allow the tribunal to see what she was like.