Most books are a combination of show and tell. The quick and dirty definition is this: showing is what you see in a movie, actions happen and you see them. Telling is narration, explaining it in short.
Saying someone is angry is not unacceptable. Showing that that they are is harder, takes more time and sometimes doesn't serve the story better.
I think writing is a bit of a combo platter. Books, as a medium, have an advantage over most movies, in that they can "tell" rather than show. Although some movies, like The Shawshank Redemption (which was, after all adapted from a book), include narration that tells.
Here's a bit of writing from what I am doing, which includes telling:
The wood for the door had come from a fleet of fishing boats, well past their prime. Generations ago, some entrepreneurial soul managed to make money on the half-wrecked fleet, cutting wood for doors before the hull gained curve, and fashioning the rest into furniture of all types--mainly rocking chairs. Not every door with a colored stripe had been reclaimed from a ship. In more manicured parts of the Isle, where wealthy retired shipmen ended up, in charming little villages, surrounded by garrisons fully stocked with private security, there too, were doors with stripes, only because the owners thought a proper door in the isles had to have the stripe. But here, amid the rough warehouses, the door with the red stripe remembered the sea. Or it would have, if sections of wood had any method of remembering things.
In this case, I've used the history, the telling of it, to give the reader a taste of the culture in the Isles. You can use telling to show something.
Motivations are interior, so I absolutely understand having to tell those rather than show them.
Telling can weaken a story if it is used too much. I use dialogue, which shows, to give readers an idea of who the characters are, but I also sprinkle in telling, to ground it out in motivation. Couldn't do a block quote for this one. Everything that's "telling" is bolded.
Tribelia sat outside the cave, by a small fire, trying to count the number of bandits in the camp. One of the men walked towards her, two bowls in hand.
“Here’s the usual stew. Not exactly palace fare, but--it’s good anyway,” Jinn said with a half-smile as he handed her the bowl.
Tribelia took it, warily, remembering that Jinn was the one who’d searched her for coin. She looked distrustfully at the brown goop. She hadn’t eaten since arriving at the camp.
“Do you think it’s poison?” he asked, his smile widening. “Poison is expensive, my lady. You’re not so hard to kill that we’d need to do that.”
She relaxed after he said that, studying him as she took large bites of the stew. She saw a man whose face, at first, appeared youthful, boyish, even, but there were crinkles at his eyes, sandy curls laced with grey. He took her scrutiny well, not seeming to care.
“We didn’t get introduced before,” she said “I’m Tribelia.”
“I’m called Jinn. Pleasure to meet you, my lady.”
“It’s nice that somebody in this camp has manners.”
Jinn didn’t say that manners don’t cost anything. Instead he said, because it was true too, “I aim to please, my lady.”
“So,” she said conspiratorially “What’s it like, being a bandit?”
“It’s like camping, only with more knives,” he answered.
“That’s clever, but it isn’t an answer,” she said, cocking her head.
“No, but it is, my lady. You’re asking--‘What’s your life like? Summarize it now.’ And I did.”
“I was hoping for something longer than a sentence, of course,” she said.
“You should stay and find out, my lady. Be a bandit for a turn of the moon.”
“I wouldn’t want to do that until I knew what it was like, besides, I didn’t enjoy getting stripped of all of my coin,” she said with dignity.
“Not all of it. You still have some sewn into the the leather of your riding boots,” he said blandly. When she looked appropriately disquieted, he said,
“Don’t worry, my lady, your secret’s safe with me. But, tell me--what’s the life of a palace princess like?”
Now I could go to the trouble of intricately describing "Appropriately disquieted" by describing the character's face, but this communicates exactly what I need. Showing that would get in the way. I don't have to show it, nor do I have to show "distrust", or the physical motion of Tribelia relaxing. You'll notice that adverbs tell, a lot, and there are many, many lists of tips and tricks advising you to cut all the adverbs and use action to show instead. I think that you should not overuse them, but in the interest of pacing, sometimes, you have to tell and you've got to use an adverb.
Too much "showing" lengthens writing unessarily or it ends up turning into a movie script rather than a book, with dry descriptions of a scene and pure dialogue.