いろは歌 is an ancient Japanese poem which contains each sound in the Japanese syllabary. This is similar to "The quick brown fox jumps over the lazy dog" which contains all the letters in the English alphabet, except the いろは歌 doesn't repeat any letter and the いろは歌 is sometimes used as an ordering of the kana.
This is an updated article originally by ち～よん.
いろは歌 is an old poem said to have been written by 空海（くうかい 774~835）who was a famous Japanese monk in the early 平安時代（Heian Era 794~1185）. According to Wikipedia, it is now thought to have originated later.
It is a poem with all the letters (like あいうえお), but it has meaning.
Because it is an old poem, the way it uses the かな is different from now. There are also many rules that are different from modern grammar.
- Some don’t have 濁点（dakuten – the voiced consonant mark that looks like a quote ゛）
- 匂えど is written as 匂へど
- ん is written as む
- きょう is written as けふ
- 酔い is written as 酔ひ
- Some old letters are used (like ゐ, ゑ), etc.
A breakdown of each sentence, how to read it, and the meaning of it in Japanese and English:
The colors of the flowers are so beautiful and fragrant–like a person’s beauty or the interesting things in this world.
色(いろ) here is the color of flowers, but it has also the meaning of the affairs of men and women, or the many events of this world. These flowers, despite their beauty today, will disappear. My life is also ephemeral just like these flowers. There are many things I enjoy in my life, but they all have an end.
（わがよ だれぞ つねならん）
My life is like that. Who can say it won’t long last forever without change? No, nobody can. It ends at last.
Some says わが世 is “I rule my world” and the author is the one who ruled that era.
I pass over the deep mountain called 宇井（うい）in 京都（Kyoto） today.
He was a 武士 (ぶし samurai warrior) and he decided to leave his life as a 武士 to become a monk to go to the temple in 高野山（こうやさん Mt. Koya in Wakayama prefecture). He travels by foot passing over the mountain in Kyoto. He leaves behind the 武士 life because he has an unendurable angst in his life. I don’t know what it was, but I imagine it may have arisen from too much killing, or his love affairs, or the struggle for power within the 武士 system, etc.
有為(うい) is also the word which means to wake up to the true reality; to remove from being a slave of mutable matters in our daily life which he compared to flowers. That is to say, to attain enlightenment in Buddhism.
So, "passing that mountain" means, he chose to become a pupil of Buddhism, leaving his enjoyable life behind, but having sorrow, too.
今日 also rhymes with 京of 京都.
（あさきゆめみし よいもせず きょう）
No more shallow dreams; no more wanton drunkenness.
I passed over the mountain of life, but it was like having a shallow dream or being drunk. But now that dream no longer intoxicates me anymore. I’ve cleared up worldly desires, feel peace and the state of enlightenment.
It is really difficult to stop being anxious about all desires and greedy feelings in our lives. But I think he thought on how to accomplish that when making this poem.
It is a very short poem, but it contains many ideas. The New Year's card game Karuta is based on this ordering.