(I was supposed to do this yesterday, sorry.)
Telling - directly stating description, thoughts, emotions, etc.'
Showing - indirectly stating description, thoughts, emotions, etc.
Like with the rest of writing, there has to be a balance. You can't have too much telling and not enough showing, or vice versa. It gets tedious after a short while. Showing too much can be confusing, when you're not spelling it out for people. But first, I'll give you an example of each so you know just exactly what I mean by show vs. tell.
"She yawned as he continued his tirade. Leaning back against the headrest, she struggled to stay awake as her brother ranted, his hands fluttering about in agitated gestures."
"She was bored as he continued his tirade. She tried to hide exactly how bored and tired she was, because she knew how enraged her brother was over this particular topic of conversation."
Easy to tell the difference between them? Showing tends to use more action, or smaller clues in the action, to state what is going on. Telling comes straight out and says it. In both, you can tell that a) she is bored, and b) he's angry. These are really easy examples, but you get my main point.
Imagine an entire paragraph in telling. I'm not going to write one because, well, it was frustrating to write the example above. I'm more of a showing kind of person. But showing gets convoluted, and if it's too difficult, your readers are going to set it down and wander off to their fridge to drown their confusion in chocolate milk. So. How do you create a balance?
Relax. If you try to force a scene to show or some dialogue to tell, it'll be obvious. If you're more inclined to showing, like I am, simply drop some telling lines every now and then. If you have to, create a stupid character and have your MC spell out everything that's happening for them. If you're a telling kind of person, then try to trust your reader; some things they can figure out on their own, like if you say the killer was tall and pale with a British accent and a person who fits that exact description is the MC's best friend. At least include actions with the emotions you're saying, like "he said excitedly" or "I lean forward in anticipation".
Watch your words. When you want to create some suspense, don't tell the reader everything they need to know. Show some of it through the characters' actions. In their words, conceal what could be interpreted as a threat. A very famous example being: "I'll be back." (You can find a variation of that line in so many books, as well...) If you're writing a slow, romantic scene, then absolutely tell more than show (especially if showing includes not enough clothing, but that's my opinion). Include emotions, like "I was nervous as he leaned in..." or some other such thing.
Take your time. Do I really have to say much on this one? As writers, you should know better than to rush through your best scenes. Don't write words that feel wrong. Don't put "emaciated" where you mean "skinny". Don't tell me he's eager to start training if he's sitting there with a blank look on his face. Don't tell me her mother is happy if her arms are crossed and she's scowling. Show what you mean. Tell us want you want us to know. Slow down, occasionally watch the words you're putting on the screen.
All in all, showing vs. telling: find a balance. Don't let either dominate your writing, or it'll be tedious/confusing. Good night, and keep writing! <3