In a nutshell, the native Japanese version of "A B C."
Japanese writing uses two syllabaries, hiragana and katakana, collectively called kana. Both are formally based on Chinese ideographs, but they are purely phonetic and hold no other meanings.
I ro ha is an old Buddhist poem that, by coincidence or intention, contains each of the kana characters exactly once. It has been used as a convenient method of alphabetization (inasmuch as a syllabary can be alphabetized) for decades. The poem, in classical Japanese, is:
The sequence is:
i ro ha ni ho he to
chi ri nu ru wo
wa ka yo ta re so
tsu ne na ra mu
u wi no o ku ya ma
ke fu ko e te
a sa ki yu me mi shi
we hi mo se su n
Fr. Francis Drohan translated the i ro ha poem in his excellent Handbook of Japanese Usage (Tuttle, 1991): "The flowers that bloom today so sweetly wither and fall. Our human life, too, is fleeting. Today, again, I will cross the mountain pass of this uncertain world, and will not entertain shallow dreams or give way to drunkenness."
The i ro ha is slowly fading from use, largely because it includes two kana that aren't used in modern writing (we and wi). The a-i-u-e-o order is more prevalent today, but the i ro ha order is still not uncommon.