You hear it all the time, but it's still one of the mistakes I see most often with beginning writers. "Show don't tell."
What does that mean? I'll start by saying what it doesn't mean. It doesn't mean only use action. It doesn't mean you can't use description or internal dialogue. It doesn't mean everything has to be visual. What it does mean is that every time you find yourself telling the reader what is happening, stop and ask yourself if it would be more effective to show the scene to the reader. For example, here's a scene where a football player walks onto the field for the first time.
Steve stepped onto the field and the crowd cheered. He was so nervous he could hardly believe it. It was a sunny afternoon and the players were catching balls and stretching. The coach noticed Steve and told him to throw some balls.
This is 100% telling. Instead of letting the reader see the scene and feel the emotions you have stopped the story to describe what is happening. Notice how this next version feels more like you are in the scene.
The roar of the crowd filled the tunnel before Steve had even reached the entrance to the field. "Fal-cons! Fal-cons! Fal-cons!" How many people were in the stands? Forty thousand? Fifty? Steve took several deep breaths before wiping his palms on the front of his jersey and leaving the tunnel.
Stepping onto the freshly cut grass, he blinked at the afternoon sun filling the stadium with the brightness of a thousand spotlights. This was it. This was the big time. "Nice pass," shouted a wide receiver as he plucked the pigskin out of the air and darted toward the end zone. The wonk of cleat on leather sounded from his right as Jeff Savage booted a football through the uprights.
Coach Wells lifted his sunglasses and spotted Steve. "'Bout time you showed up. Get over there and start throwing some balls."
We still communicate the same information. But instead of feeling like someone is telling us a story around a campfire, we are "seeing" it happen right in front of us.
The reason beginning writers do so much telling is because it's easier than showing. Telling only requires you to say that Steve was nervous. You might even say. something like, "Okay, coach" Steve said nervously. Still telling. Those are the easy way out. Showing is harder. You have to let us see Steve's actions, hear his dialog or his thoughts, and give us the information so we can figure out for ourselves how Steve feels.
Showing is harder than telling, but it's what makes the story come a alive in the reader's head.