This a technique for teaching the skills of public speaking. It is the process of showing an audience something and telling them about it. Sometimes you will bring the object from your home and some others your teacher will give you an object and ask you to describe it. There is no reason to get nervous because all you have to do is describe the object by indicating its color, what it is used for , indicate why it is special for you and how did you obtain it.
Next time that you hear that you have a SHOW and TELL don't panic take it easy, relax and do your best