Meaning of if statement without == operator

If a = 5 and the variable fl = 0, what happens in the following case?

if fl
   if a < 10
      a = a + 1
   else
      a = a - 1
   end
end

I don't understand what the if fl part means. I read that as if 0 or if false. That doesn't really mean anything to me. if fl == 0 would make more sense to me.

So what does if fl mean?