This isn't just an idiom. It's the truth. Your reputation precedes you. Why? Because it does.
Everyone has a reputation that is already known by your colleagues (and acquaintances). Your reputation is what people think about you based on two very simplistic questions:
Do I like you?
Can I trust you?
Let's start first with likability. There are usually two opinions on this topic - likability doesn't matter in business or it is does matter. I tend to land with the latter. People want to work and do business with people they enjoy. Your likability is directly influenced by your warmth, how you engage and treat others and most importantly, your level of authenticity. It's the initial barometer people are using to measure your likability.
Second, where likability serves as the foundation of your reputation, people's trust in you cultivates it. People may like you, but if they can't trust you, your reputation is nothing more than a nice person that no one would pick to be on their team. Trust, in this case, refers to your ability to show up for yourself and for others. Simply put - do you show up on time and prepared? Do you do as you say you will do and do it well?
It really is that simple. Or is it? While the way your reputation is formed or assessed is simple, knowing what your reputation actually is, isn't always so easy. Your reputation is what people say (and think) about you, especially when you're not in the room. I have often found that there is a significant disconnect between people's true reputation and what they think it is.
The best way to know what your reputation is, is to ask. To do this, I always recommend to professionals to assemble a group of colleagues and/or friends (maybe even your boss!) that will give it to you straight. A few questions to ask them:
What's the first impression that I make?
What skills do I possess that are most valued by you (or the organization that you work for)?
To your boss: Who impacts my reputation and how does that hurt or help me?
To your boss: What mistakes have I made that I need to do damage control on? What have I done well that has propelled people's impression of me?
What is important to remember about reputation is that simply ignoring it or thinking that because you don't know it, it doesn't matter. Your reputation, good or bad, could be the reason you land that next job or the promotion you've been vying for. You don't have to like it, but you should seek to know what it is. It is then up to you to decide if you want to do anything about it (tips on how to do that, for another blog post).