The quote”it is our duty to concentrate all our influence to make popular that which is sound and good, and unpopular that which is unsound”by Joseph Smith refers to the things one does and how they do it.for example if you are doing bad in school and decide to slack off then you are making bad things be popular. This means that others might see that you don't care about your education and they might start feeling the same way and will do the same thing this would be making what you do popular. Also if you are actually staying on track and not doing bad things then others will see how you are doing good and how you are set for life and will want to be exactly like you. This would be making what you are doing popular as well. And it is important to be able to influence others in a good way rather than in a bad way because this will help them and others out as well. Also if you began to use drugs then it will also become an influence on your friends and maybe even your family members. And that is something you do not or should not want for anyone