It is almost cliché to tell potential authors they need to show instead of tell. Blog posts abound on the topic. Unfortunately, people seem to little understand what this means.
The classic example is telling:
Lita was angry when she saw the mess the dog had made.
And showing:
Lightning shot out of Lita's eyes when she surveyed the remains of her stuffed animal scattered down the hallway and Rover staring guiltily up at her with a piece of fluff stuck in his teeth.
One sentence is certainly more descriptive than the other, but both these examples are not a really good explanation.
The real difference between "telling and showing" actually has nothing to do with description but rather how you tell the story. For example, an author could spend an entire chapter talking about the villain setting up a trap and going into great description about it. This would be "telling" the reader there is an intricate trap. Imagine if in Raiders of the Lost Ark, it began with people setting up a warning totem, designing the light trap and setting the spears in it, digging out the pit, attaching pulleys and levers and designing a mechanism that caused the spots on the floor to shoot darts out of the walls when people stepped on them, rolling the giant boulder into place and creating the weight triggering system, and finally setting the gold idol in place. This would tell the viewer there was a trap, and the viewer would see how intricate and dangerous it was because of the work that went into creating it.
However, this does not make for interesting fiction. Readers are not trying to learn how to create traps--although the author should have a good sense of how it was done. Instead they want to see the traps in action. That is what we get in the opening of Raiders of the Lost Ark--not the traps being set, but them being first avoided and then triggered by Indiana Jones. This is the "showing" everyone talks about as being necessary to a good book. It reveals the story by walking the reader though action and conflict.
The one sentence examples above are poor because they focus on description. Showing and not telling is more about reader engagement. I could say, "Five natives worked together slowly pushing the three ton boulder up the steep slope of the cave as sweat poured from their faces and hands." This is very descriptive, but not interesting because there is no conflict. It is only a group of people working hard to achieve a task. However, I could also say, "Five natives worked together slowly pushing the three ton boulder up the steep slope of the cave as sweat poured from their faces and hands, making it difficult to maintain their grasp." In this sentence, I have suddenly added a conflict. Instead of telling what the natives were doing, I showed that they were doing something potentially dangerous. Whether the rock slips and kills them all makes the story more interesting by compelling the reader to keep going to find out if the natives finish the job or not.
The single sentence examples are also poor because both could be found in good, engaging writing. If the plotline of the overall story is about Lita needing to get to Paris immediately to save her sister and the dog mess is one of many obstacles preventing her from catching her flight, it should be quickly dealt with and related back to the conflict of potentially missing her flight. The focus should be on getting to the flight not on once incident with the dog. The first sentence would be best in this case because this is one little step in the big picture of what is happening to prevent her getting to Paris. If on the other hand, the dog incident is the only reason Lita potentially is going to miss her flight (or potentially miss it), the second sentence would be better. Or if the whole book is about Lita's daily struggles (or her struggles with the dog) and how she overcomes them, the second sentence would be better. In determining which events to be descriptive about and which to glaze over, you need to condense the main plot of your story into 2-3 sentences and then determine how much does the event support that plot.
In Raiders of the Lost Ark, the plotline is that Indiana Jones' rival, Belloq, is always thwarting him after Jones does most of the work. Belloq joins forces with the Nazis to find the ultimate weapon: the Ark of the Covenant, and Jones must obtain it before them. Natives building the trap would not support this plotline at all. Jones going through the traps only to lose the artifact he obtained to Belloq does support it.