adverb, noun
Emphasizes that something happens on every occasion or repeatedly without exception. Slightly formal or written, but can be used in speech for emphasis.
彼は毎々遅刻してくる。
He is late every single time.
毎々お世話になっております。
Thank you for your continued support (literally: for always taking care of me).
いつも is the everyday word for 'always'; 毎々 is less frequent and more emphatic about each individual occurrence.
Reduplication of 毎 (まい, 'every') with the repetition mark 々, yielding an emphatic adverbial meaning 'each and every time'.