It's often used humorously to describe someone who is:
* Not particularly impressive or exciting: "He's just a bit of a Bruce, you know?"
* Unlikely to be successful or stand out from the crowd: "Don't expect him to be the hero, he's just a Bruce."
* A bit of a dork or clumsy: "He tripped over his own feet, what a Bruce."
It's similar to terms like "bloke" or "mate" but carries a slightly more humorous and sometimes deprecating tone.
It's important to note that "Bruce" isn't necessarily an insult. It's more of a lighthearted way to refer to someone who isn't remarkable in any particular way.