We’re overpopulated with people who consume and waste more than the Earth can sustain in the long-term.
We live on a finite planet, governed by the laws of physics and thermodynamics. Aside from physical limits on resources and energy, there is also ecology and biodiversity, which we need to maintain for things like pollination, food production, and pest control.
That’s not how the term is typically used though. I only mostly only hear “overpopulated” used by nativists and racists who demand that poor countries cut their birthrates, because they imagine the tiny proportion of their country’s national product going on foreign aid (which is practically always conditional, and effectively a subsidy to increase exports) is some terrible domestic burden that’s making them poorer.
The reality is is that rich countries, mostly in the northern hemisphere, gain trillions of dollars worth in natural resources, food, and consumer products which come from the poorest countries in the world. Within these rich countries of ours, there is also internal inequality, where a few consume more in resources than everyone else combined.
tldr; we’re overpopulated with millionaires and billionaires, who will destroy civilisation and life on the planet as we know it unless we get rid of all of them, and soon.