Q:

How do I "include the executable in $PATH"? 

I am having trouble understanding this instruction:

include the executable in $PATH: sudo cp generator /usr/local/bin

What exactly do I have to enter in my terminal? 

A:

include the executable in $PATH

This tells you what you need to achieve.   Your $PATH is the list of directories that are searched for executable programs by the shell when you try to run a command.  If an executable program is in $PATH then you can run it by typing its name. 

:

The colon is used in English to introduce something.   In this case it introduces the command that you need to type 

sudo cp generator /usr/local/bin

This is the command.   sudo means "run the command as the superuser" cp means "copy", usually where the following space-separated arguments are paths (locations in the file system) given in the order origin destination.  generator is the name of a file in the current directory and /usr/local/bin is a system directory intended to contain executable programs.  Most systems have /usr/local/bin in the default $PATH.