5 "on" (a Japanese word meaning a phonetic unit-- but in English we usually translate that to syllables)
Haikus traditionally invoke nature imagery. From Wikipedia: The essence of haiku is 'cutting' (kiru). This is often represented by the juxtaposition of two images or ideas and a kireji ('cutting word') between them, a kind of verbal punctuation mark which signals the moment of separation and colors the manner in which the juxtaposed elements are related."
Below is my favorite haiku, by Mizuta Masahide (late 1600s). In the English translation, it doesn't follow the sound form. But the haiku essence of opposition, kiru, and imagery remain.
Barn's burnt down
I can see the moon
Try one. Your haiku needn't be formal, or even serious-- but play with the kiru.