taru-adjective, adverb taking the 'to' particle
easily; simply; without difficulty
Formal or literary adverb used to describe an action done with ease, often in a somewhat elevated or written style. Typically appears as 易々と.
彼はその難問を易々と解いてしまった。
He solved the difficult problem with ease.
易々と敵を打ち破った。
They easily defeated the enemy.
簡単 is the everyday word for 'simple' or 'easy'; 易々 is more formal and restricted to adverbial use in literary contexts.
Reduplication of the kanji 易 (easy), forming a literary adverb. The reading いい is an on'yomi-based reduplicative pattern.