1.
first year of an era
The first year of a Japanese era (年号). Each time a new emperor ascends the throne, a new era begins and that year is called 元年.
令和元年は2019年だ。
The first year of the Reiwa era is 2019.
平成元年に生まれた。
I was born in the first year of the Heisei era.
明治元年は日本の近代化が始まった年だ。
The first year of Meiji is the year Japan's modernization began.
2.
inaugural year, dawn of a new era
Used figuratively to mark the year when something new begins or a major shift occurs, such as a technological breakthrough or cultural movement.
AI元年と呼ばれた。
It was called the dawn of the AI era.
電気自動車元年と言われている。
It's said to be the inaugural year of electric vehicles.
2020年はリモートワーク元年として記憶されるだろう。
2020 will be remembered as the year remote work began in earnest.
The Japanese era system (元号) assigns a name to each emperor's reign. 元年 refers to the first year of any such era. Recent eras include 令和 (2019–), 平成 (1989–2019), and 昭和 (1926–1989).
Note: 元年 is used instead of 一年 for the first year. You say 令和元年, not 令和一年.
Sense 2 is widely used in media headlines: 〜元年 marks the year something new made a major impact.