There are a hundred cards, each representing a person from Japanese antiquity, and each with an associated poem. So for example you have the Koko Emperor's poem:
It is for your sake
That I walk the fields in spring,
Gathering green herbs,
While my garment's hanging sleeves
Are speckled with falling snow.
The Chunagon Yukihara's poem:
Though we are parted,
If on Mount Inaba's peak
I should hear the sound
Of the pine trees growing there,
I'll come back again to you.
Who is still alive
When I have grown so old
That I can call my friends?
Even Takasago's pines
No longer offer comfort.
And 97 others. You can read them all here in rather nifty Japanese/Romaji/English frames; click on the numbers to get pictures.
If I was going to run a game set in Japan, I'd like to use the Hyakunin-isshu as a kind of randomiser mechanism, perhaps dictating different events for a given month or season. So each in-game month you'd draw one of the cards and, based on that, determine matters which are not directly related to the PCs - e.g. things going on between different NPCs, the weather, general success or failure or shifts in power dynamics of different groupings. That sort of thing.