This is the 12921st most frequent Russian word.
летнюю
"Летнюю" means "summer" (adjective in the accusative feminine singular form).
"Летнюю" is in the accusative case, modifying "платье" (a feminine singular noun) to describe the type of dress.
Она надела своё любимое летнюю платье.
She put on her favorite summer dress.
Here, "летнюю" is an adjective in the accusative case modifying "поездку" (trip) which is feminine.
Я всегда помню ту летнюю поездку к морю.
I always remember that summer trip to the sea.
"Летнюю" describes "ночь" as an adjective in the accusative case, referring to the summer night.