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.
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.
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.