Just what does "gain of function" really mean when it comes to this virus stuff?

EDIT: here's what I found. “Gain-of-function” is the euphemism for biological research aimed at increasing the virulence and lethality of pathogens and viruses. GoF research is government funded; its focus is on enhancing the pathogens’ ability to infect different species and to increase their deadly impact as airborne pathogens and viruses.

Just what does "gain of function" really mean when it comes to this virus stuff?