It is our duty to concentrate all our influence to make popular that which is sound and good, and unpopular that which is unsound.” (not good) - Joseph Smith

He is saying that it is up to us to make a notice of what’s right and sound good and to not make trend about what is bad. To make a trend about what’s right than we could or may influence the younger generation so they can make better choices. Why he wants us to make popular on what’s good is because he wants everybody to know what is the right thong. Something you can promote to make popular is help those in need.