When follows a character ending in the "o" sound it extends that sound. The "u" _is_ pronounced, but as "o" instead. I recommend listening to examples.
The same thing also happens with and characters that end with an "e" sound. (Eg. ). The serves to extend the "e" sound.
It is also extremely crucial to include that extension in both writing and speaking because having it or not having it will change the meaning completely. For example:
> (koukou)
Means `high school` while
> (koko)
Means `here`