To excel at or be proficient in a field or skill (often used as '〜に長けている').
彼は交渉に長けているので、海外取引を任せられる。
He is skilled at negotiation, so he can be entrusted with overseas deals.
データ分析に長けた人材を採用したい。
We want to hire people skilled in data analysis.
彼女は語学に長けているため、通訳の仕事が得意だ。
Because she is good with languages, she excels at interpretation work.
To grow old or age (archaic usage). Not commonly used in modern Japanese.
古い詩には『人は長けて知恵を得』のような表現が見られる（古語表現）。
In old poetry one can see expressions like 'people grow old and gain wisdom' (archaic usage).
To ripen (often used of fruit). Modern Japanese more commonly uses '熟す'.
秋になって葡萄が長け、甘みが増してきた。
In autumn the grapes ripened and became sweeter.
An archaic expression meaning to rise high (e.g., the sun). Seldom used in modern language.
暁に日が長けると共に鳥の声が増す（文語的表現）。
As the sun rose at dawn, the birds' voices increased (archaic expression).