noun which may take the genitive case particle 'no'
Used as a no-adjective to describe things that happen or are used on a daily basis.
日々の生活に追われている。
I'm caught up in my daily life.
I miss the days of my student life.
adverb
every day; day after day; day by day
Adverbial use emphasizing repetition or gradual change over time.
日々成長している。
I'm growing day by day.
Alternative spelling without the repetition mark; less common but still used.
毎日 is the most common word for 'every day' and is used in everyday conversation. 日々 is more literary and often carries a nuance of gradual change or reflection.
Reduplication of 日 (hi, 'day'), with the second syllable voiced to び (rendaku). The kanji spelling uses the repetition mark 々.