Very basic question: What does def do/mean?

def is short for “define”. It’s a keyword that you need to define a function (aka method). All the code that you put between the def function_name(parameters) and end will be executed every time you call the function_name later. Example:

def greet(person_name)    # begin definition
  puts "Hello, #{person_name}, nice to see you!"
end                       # end definition

greet("Alex")    #=> Hello, Alex, nice to see you!

Its stands for definition