This piece of advice comes from a tendency among writers to compose something, maybe just a paragraph or two, and fall in love with it. It might be a strongly held belief, or a favorite theory, or an old anecdote you've been saving. It doesn't matter. You love it, even if it has nothing to do with the story being told.
And then you ask someone to read your work, hoping your darling will slip by untouched, like a fastball at the knees on the outside corner. No such luck. Your trusted reader will be quick to point out the flaw. "Hey, man, what's with this business on page three? Empty calories, dude. Are you trying to inflate word count?"
Ouch! Try as you may, there is only one true remedy: your little darling has to go.
There is a story in Like a Flower in the Field that is a perfect example. The title is, "Ten Thousand Lakes." Gary and Nancy are walking through a neighborhood in Minneapolis, talking about the climate in Minnesota. Here's their conversation:
"Nancy, did I ever tell you my theory of how this region was settled?"
"No. I can't wait."
"Okay, so you know how it is in the winter, with snow up to the eaves and temperatures that drop to minus thirty or lower, and you never know if your car is going to start..."
"You got that right."
"And then in the spring, all that snow melts, basements flood, rivers overflow their banks, and you live with near-daily tornado warnings..."
"Yeah, severe storms five miles either side of a line between two towns I never heard of."
"Then comes summer with the heat and humidity--ninety degrees and ninety percent seem like the norm--plus mosquitos the size of hummingbirds..."
"The mosquito, our State Bird."
"Ah, but then comes fall, that glorious time of year. The morning air turns crisp and cold and the trees all around the lakes and down the river valleys put on their annual show, a spectacular display of red and orange and gold that is absolutely breathtaking..."
"Therefore, the first settlers obviously came in the fall. Otherwise, they would have hightailed it out of here and declared the area uninhabitable."
Nancy was laughing now, enjoying my theory...and so I went on...
Gary goes on (and on and on) to say what kind, loving, generous people Minnesotans are. He even works in a "Ya, you bettcha." Alas, it was all for naught. It didn't fit the rest of the story. I had to delete the whole conversation.
But then...I really didn't slay my darling. I saved it to share with you.