This is the 645th most frequent Russian word.
недель
The word "недель" is the genitive plural form of "неделя," meaning "weeks" in English.
Here, "недели" is in genitive plural form and used to represent the duration of time in weeks.
Я провёл две недели на побережье.
I spent two weeks on the coast.
Here, "недель" is in genitive plural form as it is the object of a phrase indicating a quantity of weeks, modified by a numeral.
Пять недель пролетели очень быстро.
Five weeks flew by very quickly.
In this sentence, "неделе" is in prepositional singular form to denote a time location in 'next week'.