This is the 18494th most frequent Russian word.
январь
"Январь" means "January" in English.
The sentence introduces January as a noun to denote a specific month in the calendar.
Январь — это первый месяц года.
January is the first month of the year.
Here, 'January' is used in the prepositional case to indicate time within the month.
В январе обычно бывает холодно.
It is usually cold in January.
The word 'January' is used in the genitive case following the numeral 'первого' (first).