千 (qiān): Thousand. A common character representing a large number.
载 (zǎi): Year; measure word for year (literary). It can also mean to carry or record, but here it clearly refers to time.
难 (nán): Difficult, hard.
逢 (féng): To meet, to come upon, to encounter by chance.
When combined, 千载难逢 (qiān zǎi nán féng) literally translates to “a thousand years, difficult to encounter.” This paints a vivid picture of waiting for an astronomically long time for a single, specific event to occur.