Kigo is a word suggesting the season the scene is taking place in. Sometimes it's obvious, as in this haiku from Basho:

The first soft snow!
Enough to bend the leaves
Of the jonquil low.

Sometimes not so obvious (Basho again):

In the cicada's cry
No sign can foretell
How soon it must die.

And sometimes it's downright obtuse (once more, Basho):

No blossoms and no moon,
and he is drinking sake
all alone!

Of course, these haiku have the correct number of syllables in Japanese, but not necessarily in English.