Let's pretend that you are playing pretend with your friends. When you pretend to be someone or something, there are two ways to do it: presentational acting and representational acting.
Presentational acting means that you are showing the audience exactly what you are pretending to be. You use your body language, voice, and emotions to make it clear that you are pretending to be a certain character. For example, if you were pretending to be a lion, you might crawl around on all fours, growl and make angry faces to show that you are a scary lion.
Representational acting means that you are pretending to be someone or something without showing it as obviously. You might still use body language, voice, and emotions, but you do it in a more subtle way. For example, if you were pretending to be a tree, you might stand very still and sway a little bit in the wind. Your friends would know you are pretending to be a tree without you having to act out every single branch.
So, in conclusion, presentational acting is when you act out everything so that everyone can see it, and representational acting is when you act in a way that suggests what you are pretending to be without actually acting out everything in a clear way.