[Opserty]

fetch_trimmed_title() is a function, you use it like any other function such as substr() or rand() or mysql_query() or something. It is just a user defined function, nothing special.

fetch_trimmed_title (line 641)
Trims a string to the specified length while keeping whole words
fetch_trimmed_title (string $title, [integer $chars = -1], [boolean $append = true])

• string $title: String to be trimmed
• integer $chars: Number of characters to aim for in the trimmed string
• boolean $append: Append "..." to shortened text

That tells you what variables you put into the function. (It is the same as when http://php.net/substr lists what parameters are for the substr() function)