This is the 4999th most frequent Russian word.
летней
"Летней" means "summer (adjective, feminine form)."
In this sentence, 'летней' functions as an adjective in the instrumental case, describing the word 'погодой' (weather). It translates to 'summer' and refers to the season, indicating the type of weather.
Она наслаждается летней погодой.
She is enjoying the summer weather.
Here, 'летней' also acts as an adjective in the genitive case, describing 'школе' (school). It conveys the specific type or theme of the school, which is 'summer.'
В этой летней школе дети изучают науку.
In this summer school, children study science.
In this case, 'летней' modifies 'одежде' (clothes) in the prepositional case, describing the style or season associated with the clothing, namely 'summer,' and providing context about the appearance.