|Picture Credit: http://blog.aacriminallaw.com/drug-offenses/|
The idea is to explain something so that readers will understand the feelings of the characters and what is happening in the scene without informing them point-blank.
Think about real life--or even good movies, for that matter. What makes you recognize an emotion? It's almost never the fact that a person spells it out for us. What specific things are happening that make you recognize an emotion?
Let's look at an example that fits in with the fact that today is Halloween:
John was afraid of the mummy. He wanted to get away, but he was trapped.--Telling
John swallowed hard and pressed tighter against the smooth wall. The mummy advanced one step after another, it's yellowed bandage hanging so that it's leathery skin poked through. John's eyes searched again for any opening or crevice that would save him. But the wall's polished surface didn't change. He couldn't keep from looking back at the monster, and he gasped at the air that no longer seemed breathable.--Showing
When you explain a few specifics about what John was doing, you no longer need to state that he was afraid, that he wanted to get away, or that he was trapped. The same is true if you want to say that a forest is pretty. What specific details does the main character notice that makes him or her think the forest is pretty? For that matter, why is the main character noticing those details?
Giving every possible detail would make for an insanely boring book--there is a difference between the dry facts of a text book and a novel. Choose the specific details that your character notices based on who they are, what they're doing, what they're thinking about, and their past experiences. A handful of specifics let the reader get caught up in the drama and FEEL what the main character feels. A list of the character's emotions is too sterile to make a reader feel what you want.
Basically, the rule is to filter everything through the main character's head. If he or she is happy, explain to me what happy feels like. What does it look like in the things around him or her (the way the things are perceived)? It doesn't matter if you are describing scenery, the main character, a side character, or anything else. The description should help the reader understand what is going on in the main character's head.