Haiku is a form of poetry that concerns itself less with rhymes and more with syllable counts. The haiku originates from Japan. Classical haiku are arranged in three lines of 5 syllables, 7 syllables and 5 syllables. Other formats do exist.
They are short, but that does not mean they are easier to write. Sometimes containing an idea in such a small amount of space can be very challenging. Some haiku contain multiple stanzas of the same syllable scheme. So...
When five syllables
Precede another seven
Then five, it's haiku
Now you do one.