na-adjective
Describes something very sweet or sugary, often with a childish or affectionate tone. Commonly used for food and drinks, but can also describe overly sweet behavior or attitudes.
このケーキは甘々で、ちょっと食べすぎちゃった。
This cake is so sweet, I ate a bit too much.
彼女は彼に甘々な態度をとる。
She acts overly sweet toward him.
甘い is the standard adjective for 'sweet'. 甘々 is an emphatic, reduplicated form that intensifies the meaning and adds a casual, often childish or affectionate nuance.
Reduplication of 甘 (あま, 'sweet') with the iteration mark 々, creating an emphatic adjectival noun. The exact historical derivation is uncertain, but the pattern is common in Japanese for intensifying or adding a playful tone.