- Better working conditions (possibly including more respect and higher social status)
- Different work (sick of development, want to move into management or product or whatever)
- Better prospects for future employment ("when I'm a middling-quality 45 year old developer I might not be able to find a job anymore... as a middling-quality 45-year-old manager, on the other hand, I'll do just fine)
One or (probably) more of those are what people want when they talk about wanting advancement opportunities. The comp is pretty much always going to be one of the things they're focused on, as it's a big part of what distinguishes advancement from changing careers. The better pay and/or status is what makes it advancement.
The only thing that is almost universally true about "advancement" is more compensation. So, if someone is complaining about advancement in their exit interview, it's very often code-word for compensation.
Which is sometimes the trigger for leaving: "why did they get the promotion when I have been-here-longer/work-harder/do-better-work?"
I'm pretty sure that's code for better compensation and better working conditions.