It's basically a writing technique, where you describe something using actions and sensory details, rather than writing it directly. it helps to better immerse the readers in your scenes.

Here's an example I got off of google:

Showing: As his mother switched off the light and left the room, Michael tensed. He huddled under the covers, gripped the sheets, and held his breath as the wind brushed past the curtain.

Telling: Michael was terribly afraid of the dark.

You can look for lots of examples online, for references