"Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen." NIV translation
As part of being the light, we are to control what comes out of our mouths. A big part of that is watching what enters our ears and eyes. If you watch things on tv or the internet that are unwholesome, they will penetrate your heart and that is the type of thing that will come out of your mouth. It is the same with what we read, video games you play, or movies you watch. This "unwholesome talk" is not just cursing but also gossip or anything else that does not build someone up in their faith.
When we listen and follow the leading of the Holy Spirit, we are able to be the light in this world. When we fail and sin, that grieves the Spirit as God only wants the best for us and when we do not listen we miss out on the best.
"Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice." NIV translation
These are not things that are consistent with being a disciple of Jesus Christ. We must remember that Christ died for us even while we were still His enemies. Instead of showing these things to us (which we deserved) He gave His life for us.
"Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you." NIV translation
It's not enough to get rid of the old self and the old ways. We are called to put on the new self and the new way which is love. Because of Jesus' love for us, we are able to know and show love to others. Part of that love is forgiveness just as God forgave us. This is not a temporary thing like we sometimes do but is a permanent getting rid of what someone has done to you just like it never even happened. That's what God's definition of forgiveness is and ours should be the same.