What are haikus usually about?

Traditionally, haikus in Japan reflect nature, but modern haiku in both Japan and abroad deal with a variety of topics;

Haikus can be about everything (1), anything (2) or nothing (3). The key to a haiku is the measure, not the content. It does not even need to rhyme.

(1)

Everything, or

The entire Universe,

Reflection of God

(2) A Haiku poem can be about a season, for example:

Autumn is now here

red, orange, yellow and brown

leaves are falling down

(3)

Bored, and I can't write

a good haiku anymore.

I'm going to bed