I am initiating and doing a basic penetration testing course and have come across one doubt.

For example in this URL: http://webapp.thm/index.php=?page=1

I get that I am requesting the index.php file in the server and I am specifying that I want to view page 1 as a value of parameter page.

But in this other URL: http://webapp.thm/get.php?file=/etc/passwd

I am having trouble understanding this request because I am requesting get.php(which is already a file) and the parameter "file" is accesing a different file /etc/passwd.

My question is what does the get.php file contain in this case or is it a function? because it seems to me that it is function retrieving the /etc/passwd, but I know that it is not a function, so I am kind of confused about what could the get.php mean, here I got a picture describing it from the course  

I am initiating and doing a basic penetration testing course and have come across one doubt.

For example in this URL: http://webapp.thm/index.php=?page=1

I get that I am requesting the index.php file in the server and I am specifying that I want to view page 1 as a value of the parameter.

But in this other URL: http://webapp.thm/get.php?file=/etc/passwd

I am having trouble understanding this request because I am requesting get.php (which is already a file) and the parameter file is accesing a different file /etc/passwd.

My question is what does the get.php file contain in this case or is it a function? because it seems to me that it is function retrieving the /etc/passwd, but I know that it is not a function, so I am kind of confused about what the get.php means. 

got a picture describing it from the course: