This is the 12920th most frequent Russian word.
летнее
"Summer" or "summery" (adjective).
Here, 'летнее' is used as an adjective to describe the noun 'солнце' (sun), indicating that it pertains to the summer season.
Летнее солнце согревало землю.
The summer sun warmed the earth.
In this sentence, the word 'летними' is a plural instrumental form of the adjective describing 'днями' (days), referring to days of the summer season.
Мы наслаждались летними днями на пляже.
We enjoyed summer days at the beach.
In this example, 'летнее' is again an adjective describing the noun 'платье' (dress), specifying that the dress is suited for summer.