The phrase "massive swarms" means a large group of things or animals coming together or moving together in a big way, like a big crowd. Full definition
Brought to you by the studio who collaborated with 343i on Halo: The Master Chief Collection and Halo: CE Anniversary, the game features massive swarms of zombies which rush to their living prey. (wccftech.com)
Featuring massive swarms of zombies who rush their living prey, World War Z is in production for Xbox One, PS4 and Windows PC. (xboxgamerreviews.com)
It often forms massive swarms, some as much as 100 kilometres long. (newscientist.com)