"Telling" is summarizing the action for the reader:
He was nervous. She was angry. The shark was scary.
The reader doesn't experience the action directly because it's filtered through you, the writer.
"Showing" is writing the action directly as your character experiences it.
His hands got clammy. She clenched her fists. He stopped breathing as the shark circled.
The reader experiences the action directly, too.
Sometimes we need "telling," to keep the story moving. But in most cases, "showing" will make your character's actions or feelings more vivid.
Try rewriting these examples of "telling":
Bob was happy.
The girl was serious.
The cake was stale.
The room smelled bad.
Here are my rewrites. How about you?
Bob's grin covered his whole face.
The girl stared at him with unsmiling eyes.
The dry cake crumbs stuck in his throat.
When he opened the door, an odor hit him.
Try "showing" instead of "telling" when you want your readers to feel exactly what your character is feeling. Sometimes it's okay to "show-off!"
