Voice is the ability to channel your personality into writing.
If you are reading something, and it feels like someone is talking to you. That is voice. Basically the writing has to make you feel something. Does it make you laugh? Does it make you feel passionate? Does it anger you? Inspire you? If you are ‘moved’ by a piece of writing, then it contains the author’s voice. Mission accomplished.
Writers do all kinds of things to suffocate their own voice. They try to sound intellectual. They write in a way that would impress their 10th grade English teacher. They try to use bigger words than are necessary. They fall prey to a variety of tempting hacks that prevent them from putting their words down naturally. If you write to please someone else, no one will feel your voice. It will seal your words in an irrelevant vacuum.
Voice is letting your personality flow. There are no rules to establishing your voice, because everyone has one that belongs only to them. It’s not about swearing a lot, or talking like a gangster, or trying to write like a character in a film you love. That will only lead to imitation. There are things only you would say. Things only you would think of and topics only you would connect. I cannot tell you what those combinations are. Neither could your tenth grade English teacher. The only person who can unlock your voice is you. And the only way to do it is to write constantly.
One reason I love to read a diversity of writers is that it let’s me know just how many damn voices are out there. The more I read, the more I believe in the power of voice, and the more I trust that I have a unique voice of my own. When you write with the confidence that you are being demonstratively you, your voice will spill out. It will be uncontainable. Keep doing it. Your voice will get you heard. Your voice, your unique ability to make people feel something with your personality is the most powerful gift you have.