The opening verse, the hokku, has to have a seasonal word. Let's say it anchors that opening verse in a particular moment in time. So, if there's the word hototogisu, the cuckoo, it means that this is being composed in summer.
through a vast bamboo forest
moonlight seeping 1
Every haiku has two parts to it. It's divided in the middle by what's called a "cutting word". It's a structure that is designed to engage the reader and it permits multiple interpretations to this potent poetic form.
karasu no tomarikeri
aki no kure
on a bare branch
a crow has alighted
autumn evening 2
The kigo, or seasonal word, is very obvious: it's the autumn. And there's what's called a kireji, or cutting word, in the middle and it comes right after "has alighted," "tomarikeri." So we have two parts to what's now called the haiku, but what was then called the hokku. "On a bare branch a crow has alighted" and then there's a break, and the second half is "autumn night fall" or "end of autumn."
Now, the important part about the cut, the kireji, which cuts the two parts of the haiku is that it leaves the poem open for the reader to complete. So, it's like the linked verse. You have one verse, the verse is basically unfinished. The next person has to complete that by adding a verse. The same thing happens within the bounds of the haiku, or the hokku. The two parts are sliced in half and there's an open space which the reader, the audience, is supposed to enter into.
1. Makoto Ueda, Bashô and His Interpreters: Selected Hokku with Commentary, p. 314.
2. English translation provided by Haruo Shirane.