Where's the description of "@startuml"

Question

Hi,

I'm looking for an explanation of "@startuml".
Where's the description of this writing?

@startuml{Fig4.1.png}

or

@startuml{Fig4.2.png}

I expected it to be created with the name written in parentheses,
The only figure created was one named "Fig4.png".

Thank you.