Despite your question asking about darkness, I will probably speak more about light. Because darkness and light interact (or I suppose do not interact).
Darkness, as the other answers have already stated is the lack of "light." And this is light, as described by Psalm 119:105.
Your word is a lamp to my feet and a light for my path.
So in a sense, darkness is like being lost. That type of lost that describes not being physically lost, but lacking a direction and a purpose in life. Or maybe there is a direction, but it's the wrong direction. And God's word, is a light that guides us back to the correct path and directs us out of the darkness and towards God. This is still quite surface level. Let's go a little deeper.
As mentioned by the other answers, the word "darkness" is often used in the bible to describe a life of sin, a life that rejects God. If you think of the word darkness, and think about real life physical darkness, it is often very hard to see in the darkness. And in the darkness, you can hide a lot of things that you do not want people to see. (In the case that we're talking about, these 'things' are our sins.) stevenl mentioned Ephesians 5:7-11. Particularly, I want to look at Ephesians 5:11.
Take no part in the unfruitful works of darkness, but instead expose them.
God's light purges the darkness and reveals sin. However, I'd like to add that this is not done in a condemning way, but rather in a way that will lead people to God and that will glorify Him rather than in a way that turns people away from God, as stated also in a verse that stevenl mentioned, John 3:17.
For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him.
We do not share God's light with the world to condemn, but rather to save.
So in conclusion, (there's kind of a bunch of off-topic stuff as well, but it's necessary) living in darkness is essentially to us, being lost. The people who are lost do not think that they are lost. They believe that the direction that they are chasing will lead to happiness and joy. The direction that they chase often leads into sin and disappointment, sometimes causing them to become more lost. We however, know that the true direction is to chase after God. That path should lead us away from sin (though we sometimes stray). When we interact with those who are "in darkness" we should be revealing the sin in their lives to them, not in a condemning way (this is hard), but in a way that leads them to the same path that we're on, into repentance.
I hope this answer helped! If you need more clarification about something, please do not hesitate to comment!