When a friend returns back from an adventurous trip from the Sahara desert in Morocco, or the Road of Death in Bolivia, would you rather your friend tell you how she felt during the trip (in a non-literary way; "I had so much fun, the sand was everywhere and the camels run like crazy" etc) or would you rather your friend show you the videos, pictures, or better yet, bring you along with him/her in her next itinerary trip?
The latter is the better option for most people, especially the creative ones, because you become part of the action. You can either feel or empathize with your friend's feelings or make one for yourself as if you were there -- or both if you're as reflective as I am. What this does is that it brings pulls the audience from their viewing spot and allows them to experience something outside their reality; to be lost and into the moment.
Consider the difference in these two examples:
...and I feel overly reflective and sentimental
Lowering my head and watching
the three candles before me
decorate the light of the room
I hope you forgive me of this gesture.
...and the way these three candles--
each a different height--
are singing in perfect harmony.
So forgive me
if I lower my head now and listen
to the short bass candle as he takes a solo
while my heart
thrums under my shirt--
- excerpt from: "I Ask You", Billy Collins
Both examples show the same scene (a man reflecting), but the former merely tells the audience what is happening; like a blank picture, while the latter is more engaging, as if you are one with the speaker. "Showing", obviously has other elements of poetry that belongs to another article: mainly, (i) imagery, and (ii) metaphor. The metaphor is that every reflection is like music (it could be a sad beautiful song, angry, happy, etc), and the background surroundings of the speaker is used as tools to create the metaphor and then the imagery, which is the sound of the candles as they flicker, and the heart of the speaker joins the flickering.
The audience is engaged to participate with the poem instead of just reading it. We, as an audience, get to interpret or reflect about the speaker, "Why is he/she asking for forgiveness?", "Is he in an apartment? Or at a lake house somewhere?" etc.
There is nothing wrong with telling, don't get me wrong, but it is always best to try and avoid using it in a poem (or a story) unless you have to. Like in film, voice-overs are a very uncreative way of describing what the character feels. If he feels the heat, it lacks creativity if a voice merely narrates, "Oh, it is a hot day today. I am sweating like a pig." (unless that's part of the characters personality, etc, but...) Instead the director must "show" this feeling by focusing on the character's sudden movements (does he have the posture of a pow?), reactions (is he easily irritated with noise), expression (maybe his shirt is being stained by the beads of sweat dripping from his brow) -- whatever it takes for the audience to feel the heat instead of declaring, "Hey, you people, watching from you seats, understand that I feel hot because I said so."
As writers, we aspire to express and gather various personal interpretations or conclusions from our readers. By "showing", we merely give clues that aren't too vague but aren't too obvious to succeed with our aim. So be creative, engage yourself in experience, and remember that every poem has its own story, its own emotion, give it a chance to show itself.