Q:

What does the lone question mark symbol and lone colon symbol mean in Ruby

In this snippet of previously written code, there are ? and a : used by themselves (lone symbols), and not appended onto a word. What do they mean in this instance?
  # Fill application object with applicant info; generic info if not provided.
  availability ? formdata[:availability] = availability : (formdata[:availability] = [0,1,2,3,4,5,6])

A:

This is the ternary operator. It is the equivalent of an if/else. Your code could alternatively be written like this:
if availability 
  formdata[:availability] = availability 
else
  (formdata[:availability] = [0,1,2,3,4,5,6])
end

I would also argue that it should be simplified to something like this instead:
formdata[:availability] = availability ? availability : [0,1,2,3,4,5,6]

The assignment in the conditionals is an odd way of writing that, and not common.
You could also simplify this further to:
formdata[:availability] = availability || [0,1,2,3,4,5,6]

The details of how an || (or) assignment operates can be found here

