It's often used to describe someone who is brave, courageous, or has a lot of guts. It can also imply that someone is confident, bold, or assertive.
Think of it like saying "He has a pair of brass ones" or "She's got balls of steel" in English.
It's important to note that "cajones" is considered a vulgar term in Spanish. It's often used in informal settings and may not be appropriate in all situations.