What is the meaning of the dialogue at the end of the episode 30?
It's quite literal; as you said, Stain, though unconscious, remained standing, while several of the present heroes had fallen over in shock (Shouto and Tenya fall to their knees, one pro hero completely fell backwards, and even Gran Torino and Endeavor are taken aback) upon witnessing Stain's bloodlust.

(The narration also states that at the moment Stain froze, one of his broken ribs pierced his lung; I personally headcanon that this internal bleeding triggered Stain's own quirk upon himself, but this is by no means confirmed.)