taru-adjective, adverb taking the 'to' particle
slight; insignificant; trifling
Literary or formal adjective/adverb describing something extremely small or negligible. Most often used in the fixed pattern 微々たる + noun, or adverbially as 微々として.
See also: 微々たるもの
彼の貢献は微々たるものだった。
His contribution was insignificant.
微々として気にすることはない。
It's not something to worry about in the slightest.
Reduplication of 微 (び, 'tiny, subtle'), forming an emphatic literary expression meaning 'very slight'. The exact historical derivation is uncertain, but the pattern is typical of Sino-Japanese reduplicative adverbs/adjectives.