1 John 1:7

But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.

(v7, NIV)

So, as John mentioned in the last verse, if you say you have fellowship with God yet walk in darkness, you lie. You cannot walk in darkness and have fellowship with God. God is light and in him there is no darkness. Therefore, it is impossible for one to walk in darkness yet have fellowship with him. Now, John addresses those who do in fact walk in the light. These people who walk in the light do not walk in darkness. They walk in proximity or similarly to God since God is also in the light. If we walk in the light, we have fellowship with one another. Back in verse three, John says that the reason why they are proclaiming what they have seen and heard is so that those who hear will have fellowship with those who proclaim. All these people are walking in the light and they all have fellowship with one another. In addition, though John doesn't exactly say it here, we have fellowship with God. God is also in the light that we are walking in, so we have fellowship with him too.

Nobody's perfect. In fact, some may claim to be walking in the light, but others can point to something in their lives that are sinful. It's impossible to be perfect because we all have sinned. So how does one walk in the light? We can try to walk in the light and not walk in darkness, but sooner or later, we'll fall in sin. To walk in the light does not necessarily mean to not sin. For those who have heard and testify and proclaim the word of life, they know of the light. The question is whether they will make a practice of sinning or not. You may in fact sin while walking in the light. Fortunately, if you walk in the light, the blood of Jesus purifies us from all sin. Our sins are washed away. John doesn't exactly specify how, but we know that we are saved by the grace of God through faith in Jesus Christ. Jesus calls us to repent for our sins. Our sins can be washed by his blood. However, this is only true for those who walk in the light, not for those who walk in the darkness.