It's the infamous saying 'show, don't tell' in fanfiction. The basic jist of this is that you should show us, through visuals and descriptions, what is going on instead of outright telling us. People love being entertained, for their imagination to be stimulated, and this happens through vivid descriptions and a life-like explanation of what is going on.
Take a popular book like Lord of the Rings, for instance. The story that is being told is partly as appealing because it involves a world we aren't familiar with. A picture is being painted before our eyes of something otherworldly. Had Tolkien simply gone and said 'Well, this is Middle-Earth, these people live here, and we want the bad guy dead.' everyone would not only have disliked it greatly, they would have been confused as all heck.
It's hard to explain what it means, though, so examples might not make you understand perfectly either. Let's try, though. Say, we have two people who travel together, and one of them has a crush on the other. Character A has a crush on character B. Then character C shows up and continues to sweep character B off his/her feet. Character A then proceeds to get incredibly jealous.
So, how would you describe this? The 'tell' part of this might be:
"Character A was jealous of character C and stomped his foot."
This is a very basic example, but it's true. This is me telling you character A is jealous of character C. It's a fact given to you by me, but it's also utterly dull. Now, I could also try to show you.
"The way the two were looking at each other made character A's skin crawl. An angry scowl was etched on his face, deepening with every flirtatious smile. He tensed, staring with dark eyes at the scene unfolding before him. His hands itched to interrupt them, perhaps swing a fist into character C's face, but he was unable to move. This was something even he couldn't do anything about."
Nowhere in that paragraph did I mention the word jealousy, but it's - I think - clear the character is feeling that way. Through description of a person's actions, thoughts, emotions and using all the senses, I can evoke an idea without outright stating it.
That's what people mean when they say 'show, don't tell'.