adverb
Describes visiting a place or person repeatedly, often with a sense of dedication or habit. Commonly used with verbs like 通う (to frequent) or 訪れる (to visit).
彼は足繁く図書館に通っている。
He frequently goes to the library.
あの店には足繁く訪れる常連客が多い。
That shop has many regular customers who visit often.
頻繁に is a general adverb for 'frequently' and can be used in any context, while 足繁く specifically implies repeated physical visits to a place.
Compound of 足 (foot) and 繁く (frequently, thickly), literally 'with thick feet', implying frequent comings and goings.