A haiku is a three-line poem with a syllable count of 5 on the first line, 7 on the middle, and 5 on the last line. For example:

North in the Gold Coast
I see a familiar sight
Beautiful Anvil

Hidden cave grows life
Vvardenfell moon sugar here
Do I help Khajiit?

Werewolf skill grows strong
Initiation with bite
Dreams of Cyrodiil

An easy way to count syllables is to count the number of times your jaw moves downward as you speak the words. For example, "Elder Scrolls" should have 3 syllables. If someone has a better method, please share!

Need inspiration? Check out this old thread: https://forums.elderscrollsonline.com/en/discussion/244781/eso-haiku

(As a side note, please do not harass people for incorrectly counting syllables, it's difficult and not so intuitive indeed, there is nothing so destructive of creativity when people are bashed for attempting to enjoy the same things you do)
Get creative!