taru-adjective, adverb taking the 'to' particle
Describes a firm, unwavering attitude or manner, often in the face of difficulty. Used as a taru-adjective (毅然とした/毅然たる) or adverbially with と (毅然と).
彼は困難に直面しても毅然とした態度を崩さなかった。
He maintained a resolute attitude even when faced with difficulties.
政府はテロ
The government should stand firm against terrorism.
The kanji 毅 means 'strong' or 'resolute', and 然 is a suffix forming adjectives/adverbs describing a state. The exact historical derivation is uncertain, but the compound is used to express a firm, unshaken manner.