Oh how those words were pounded into me over and over while I was having my very first manuscript edited. At first, the concept was a little hard to get my mind around, but with patience, in me, and some great examples, I started to catch on to the concept. Show means to place the picture into the mind of the reader rather than the author just saying it. For example, it was a great day leaves a lot out, but saying it was a warm, sunny day with no clouds and a sun-faded blue sky makes it much clearer. Whenever possible, I try to put it either in the mind or the words of my characters. Rather than just putting in the words within a paragraph, I like to have my character observe it. This helps to keep the story moving forward. Something like: Pete stared up at the dark-grey clouds and frowned. Here he was hoping to get out and chop more wood in preparation for the coming winter and the day only promised more rain. At least it isn't cold, Pete thought to himself as he guessed it was still in the sixties. Then if you want to set a picture of the time of year, you can add: Looking at the trees all around him, Pete observed the various shades of reds, oranges and yellows, with green mixed in here and there, as autumn was upon him and winter already knocking on the door.
Prior to learning how to describe things I would have probably just written; Pete stepped out this autumn day with its promise of rain and its changing colors and hoped for the best in getting some wood chopped today. Says the same thing, but I'm not really showing as much as telling. And they say an old dog can't learn new tricks - ha!