This is one of the most common pieces of advice given to newer writers. When you tell the story all you are doing is stating what happens and nothing else. You don’t write about the deeper feelings of the characters, the minor details of the setting and characters which may be useful to you later, or anything that the writer may take interest in other than the blatant events of the plot. When all you do is tell story it will be boring, this is why the common piece of advice is to show and not tell. You want to show your reader what is going on so they have a deeper connection to the story and so that they won’t read one page, shrug, and say “well that happened” before putting your story down and moving on.
Now what this piece of advice outright ignores is the fact that there are some moments where telling the story is better than showing it. In an action packed, stressful scene you don’t want to give any extra details at all even if they have a strong connection to the plot. I don’t know about you, but when I’m extra stressed out I start overlooking details and paying attention to only the basic ideas of what is going on. Characters should be the same. In this situation it is best to tell instead of show. As long as you don’t tell your story before your reader is introduced to the world and characters, telling a few scenes here and there can actually add to the emotional impact of your story.