Someone who's bold is daring and brave. You might show how bold you are by climbing onto the roof of your house, or by speaking up when you see someone being treated unfairly.
When you act in a bold way, you're taking some kind of risk; you could be risking physical danger, embarrassment, or your reputation. Whatever bold actions you take, they are confident and fearless. The adjective bold can also be used to describe a particular typeface that is thick, dark, and confident. Bold type is usually used to emphasize something strongly in writing.