Externalities refers to the economic concept of external costs:
"An external cost is a cost not included in the market price of the goods and services being produced, i.e. a cost not borne by those who create it".
The less real costs are factored into a product, the less the market able to efficiently price it. Many of our greatest modern problems could be ameliorated if we handled external costs better: pricing industrial runoff, CO2 emissions, antibiotic resistance, etc.
We mostly choose not to price in those costs, because the consequences of such are generally delayed, and it allows us greater standard of living for the moment. When given the options "meat will be cheaper but infections harder to cure in 20 years", or "energy will be cheaper but large scale climate disasters will be common in 50 years", we've chosen cheap meat and energy.