The phrase "huge swarm" refers to a large group of something, like bees or people, that gather together closely and move together as one unit. Full definition
The game is full of neon colours and creatively designed enemies, something taken from Alienation, and later levels feature huge swarms of creatures, echoing Dead Nation. (thesixthaxis.com)
Though there were no reported disease outbreaks, epidemiologists were still worried, especially when huge swarms arrived in Houston. (discovermagazine.com)
But this year huge swarms have been spotted and photographed along the coast of northern Croatia, and the coasts of Slovenia and Italy, as far south as Pesaro. (newscientist.com)