We are often told, “show, not tell” in writing. Telling is easy. You tell what people look like. You tell what they do. What is showing? Showing is using imagery and detailed description to help the reader see for him or herself.
One example of “show, not tell” is in figurative language. You could describe what something looks like, but you will not be able to get the reader to visualizing it quite as well as if you make a comparison. For example, an author can say: It was dark. However, that sentence is merely telling. You can show by making a comparison. Here is an example from “The Most Dangerous Game” of showing “dark” to the reader.
"Ugh! It's like moist black velvet."
In this case, the simile comparing the dark night to “moist black velvet” creates an impression for the reader. We can picture what the author is trying to describe. The imagery is an example of the author showing and not telling.