Haiku is an old poetic form that developed in Japan from both Japanese and Chinese poetic traditions. Haiku submitted to journals like Frogpond or Modern Haiku are from all over the world. It is popular perhaps because it is short and most people feel they can write a three-line seventeen-syllable poem. Here's one I wrote a few years ago about the funeral of a classmate and friend. The crow overhead seemed to capture my feelings.
taps from a bugle
tears in the eyes of family
crow flaps slowly past
Seventeen syllables for the haiku came into English because the Japanese haiku has seventeen sound units in it. But English syllables and Japanese sound units are nothing alike. In her book Writing and Enjoying Haiku, Jane Reichhold suggests that a closer approximation in English would be around twelve syllables.
a soft rain
spatters on the windows
it wakes me
Another alternative that haiku books often suggest is that we write three lines with the first and third lines shorter and the middle line long.
down from a leafless elm
The haiku traditionally has its basis in the observation and reporting of something from nature, although even that requirement relaxes in modern haiku. Just as Sixteenth Century sonnets were rhymed iambic pentameter, modern poetry tends to free verse or unrhymed, but metrical forms, modern haiku has relaxed many of the strictures that early writers of haiku in English followed.
Haiku are usually written about some small moment, often called a haiku moment, when something seen or heard or sensed triggers a strong emotion like joy or love or fear or sadness or even a memory that evokes strong emotion. The poet captures that image in the haiku.
For example, I saw frost on the bedroom window this morning. It triggered joy in me, seeing the frost, and it reminded me of mornings when we have had hoar frost that melted quickly when the sun warmed it.
drift away on sunbeams
I write a haiku almost every morning and trade it with my dear friend Chris, in Birney, MT, who sends one my way. We write about sunrises, sunsets, light on the trees, ice in the river, dogs barking in the night, animals or birds we've seen or heard, happy or sad events of the day or day before.
I suggest you try it. Start with three lines of around twelve syllables to capture some sight or sound or feeling. You might be surprised at how your powers of observation improve so you see and feel more. Don't be put off by rules. Just pay attention your senses and thoughts and play with them until you condense them to capture your emotional response.