Ben (Duane Jones) delivers that line in Night of the Living Dead, directed by George Romero and released in 1968. He's referring to the zombies outside of course who will continue to move forward, never stopping, unrelenting. When Harry Cooper (Karl Hardman) argues for everyone moving to the cellar Ben assures him the house is boarded up and "those things have no strength." Ah, but Cooper argues, having had his car turned over, individual strength doesn't matter when faced with overwhelming numbers. If they're enough of them, they will have strength.
So what's more frightening? The single inhumanly strong force (Vampire, Werewolf, Monster) or the weak in strength but strong in numbers pack (Zombies, Pods from Invasion of the Body Snatchers, the Borg from Star Trek)?
I vote for the pack. A single panther on the hunt is scary, like the hunter of the Predator movies, but can be defeated with a plan. A wolf pack however must be outlasted. There is no plan other than destroy, destroy some more, and keep destroying. Hopefully, before you die of exhaustion you will have destroyed them all. If not, you're dead. And that's your plan. That's one of the many reasons I believe Night of the Living Dead touched on such a nerve upon its release in 1968 (and no I'm not going to get into zeitgeist baloney about Vietnam): Because horror had for so long been based around a superhuman beast like Dracula or a mad killer like Norman Bates. But Living Dead gave us unthinking, unblinking multitudes. The fear was in the numbers. And isn't that what most of us fear the most? Do you fear a demagogic leader, or the unthinking, unblinking multitudes that will follow that demagogue and give him power? Unthinking multitudes with no plan, no strategy, just instinct. Relentless, driving instinct. It's always been a fear. It's always been a dread, deep down in those places we don't like to visit very often. And Romero was among the first to tap into it for all its horror was worth. Dracula as a character interests me and fascinates me but the living dead touch on something real. Something you can't think your way out of. You simply must outlast it. And endure.