The film is heavy in symbolism and is a display of how society can dictate to us what roads we take in life. How we may truly be someone other than the life we live.
It's broken down into three chapters: Little, Chiron, and Black. Each representing a different time in this person's life and portrayed by a different actor. (All do a fantastic job.)
Chiron doesn't talk a ton, so while he's the focus, the story gets told a lot more by those in his life. A drug dealer with a good heart, a friend who becomes his mother figure, his drug addicted mom, a best friend who isn't but is also more, and so on. All of their journeys are also key to showcasing how society helps to choose our paths. Important note: You're thinking "drama about black kids in the hood, I've seen that movie". This is not that film. Nobody is busting a cap in anyone's ass.
The underlying truth of the whole film is that we determine our own path. However, the point, imho, is how difficult it is to walk that path. Society, family, and friends are all there telling you how to live your life.
There's no traditional plot here. Just a strong character piece. One of those movies I'm glad I watched, really enjoyed, and will never watch again.
If you have no plans to ever watch it, here's a good video that breaks down the meaning in the first few minutes (they agree with me, woot!) then goes on to discuss some of the film making tricks used to tell the story. However, I recommend you just watch the film.