Archaic adverb describing a loud, deep, reverberating sound such as thunder, waves, or a crowd. In modern Japanese, the onomatopoeic ゴロゴロ or the verb 轟く (とどろく) are more common.
In classical texts, it was used like 'the thunder roars with a rumble.'
The thunder roared and echoed through the sky.
ゴロゴロ is a common, colloquial onomatopoeia for rumbling or rolling sounds, much more casual and current than the archaic 轟.
The exact derivation is uncertain; the kanji 轟 is conventionally associated with loud rumbling sounds, and the reading とどろ is related to the verb 轟く (とどろく).