Are you asking like... in a philosophical way? Or you just want to know what people mean by it?

It's just that we tend to set off in life with the goal of achieving some form of material wealth. We get lost in our careers or whatever we do to stay to keep busy and/or to stay alive. Like rats in a lab, we often don't see the big picture and, in some ways, what we want and what we're supposed to want is controlled by people who are more powerful than we are and are far out of our reach (not necessarily in a conspiracy kind of way, it's just that most of us are less powerful than actual powerful people). We feel like we're moving forward in life but really we're not going anywhere, which is obvious if you look at it from the lab worker's perspective.

I guess it's a somewhat sanctimonious way to look at, especially, the middle class life. But some people really do seem, at least to me, to really get lost in this pointless race. The race to a bigger house and a bigger TV and another gadget, some botox injections or a meal in the hottest restaurant, and never seem to really give it another thought. Maybe that's the "healthy" way to live life? Just not to think about it too hard.

Author Robert Kiyosaki also used the "rat race" as a term in his book Rich dad, poor dad, collectively referring to the things middle class people do wrong which keep them in a cycle of not achieving true wealth. He even built a board game around it which was somehow meant to train the mindset of escaping the rat race. I've never played that game. Maybe that's why I'm not rich.