Someone correct me if I'm wrong, but I believe this refers to the fact that it can be expressed in terms of certain simple mathematical operations like addition, subtraction, multiplication, powers, roots etc.—and as a consequence, the execution is very efficient. My understanding is that 'closed form' solution is essentially something that resembles a polynomial (again, accepting corrections!).
Closed form just means you can do it in a finite number of operations. So just "run X" rather than the previous versions of this kind of thing which are "repeat X until measure Y is lower than the limit I care about". (my basic understanding)
I checked the Wikipedia article, and the sorts of operations involved do appear to be a part of the definition: https://en.wikipedia.org/wiki/Closed-form_expression —though it sounds like it's a somewhat loosely defined term.
