taru-adjective, adverb taking the 'to' particle, nari-adjective (archaic / formal)
continuous; unbroken; uninterrupted
Literary or formal adverb/adjective describing something that continues without pause. Often used with と (e.g., 連々と続く) or in the たる form (連々たる).
山脈が連々と続いている。
The mountain range stretches on continuously.
連々たる車の列が高速道路を埋め尽くした。
An unbroken line of cars filled the expressway.
Reduplication of 連 (れん, 'link' or 'connect'), emphasizing a continuous, unbroken state. The exact historical derivation is uncertain, but the form follows a common pattern for creating adverbs/adjectives in literary Japanese.