Your signature style is unique to you and comes from your style personality and certain styles that you know that you wear well.
It doesn't mean looking the same everyday! Instead, people will begin to positively associate you with a particular look that is just "you." For example, you may like to experiment with different types of accessories like belts, hats, or scarves. Or you may prefer bright colors instead of more muted tones, or vice versa.
To go a step further, your signature look could eventually evolve into your personal brand. Your personal brand should stay consistent over time, for the most impact. (Imagine if Nike changed their "swoosh" symbol - after all these years!)
So what is your signature style? It may be something so subtle, that you don't even realize that you're doing it. And remember: Ideally, your signature style should work for your image, not against it!
The proud owner of my own wardrobe and home decor consulting company-- Practical Style and Spaces!