This verse in Thessalonians is very direct and to the point. It's important that we keep our hearts pure. We have to avoid evil. What we see and listen to has an effect on us. We have to make sure that the effect is a positive one.
Psalms 19:14 "Let the words of my mouth and the meditation of my heart be acceptable in your sight, the Lord, my rock, and my redeemer". We have to meditate on good things. There is an old saying "garbage in, garbage out". What that means is, if we fill our heads with evil or immoral things, we tend to say and do things that are wrong in the eyes of God.
One of the ways we can keep our hearts and minds pure is to read the Bible. Its important to read it and meditate on it and what it means. The Bible is one of the ways God speaks to us. So if we read it every day and listen, then we can get a clearer picture of how God wants us to behave. We also should make sure the music we listen to and the movies we see are acceptable in the eyes of God. These forms of media can have a powerful impact on how we as people handle certain situations and how we respond to the needs of others. Some music and movies are just bad for us. The Bible will help you to make better decisions in this area.
God has something amazing for all of us. There are areas that He wants to use us. If we are watching and listening to things that are bad for us then we won't be sensitive enough to what He is saying. We all long to be used by God so we have to stay pure and keep our minds focused on Him. If the word of God is a constant source of inspiration for us and we let it be a filter for what is good or bad then we can be a positive influence on those around us.