declare a global variable in Ruby

$global_variable = "Hello World"
``

This code is declaring a global variable in Ruby. The variable is given the name of "$global_variable", and is assigned the string "Hello World".

A global variable is a variable that can be accessed from any scope (or part of the code), anywhere in the program. To declare a global variable in Ruby, we start with a "$" sign followed by the name of the variable (in this case, "global_variable"). We then use the assignment operator "=" to assign a value to the global variable; in this case, the string "Hello World".

Once the global variable is declared and initialized, it can be accessed from any part of the code. For example, we can print out the global variable by using the puts command: `puts $global_variable` which will print out "Hello World".