The function is: f(n) = (n-th triangular number) ^ 2
So, f(1) = (first triangular number) ^ 2
the '^ 2' means "multiply the number by itself", e.g., '3^2' is '9'.
And, f(2) = (second triangular number) ^ 2
and, f(3) = (third triangular number) ^ 2
So, what is a "triangular number" ?
Do an Internet search for "definition triangular number",
and you may find the web-page;
http://www.mathsisfun.com/definitions/triangular-number.html
which gives a definition, plus pictures to help you visualize why they are called "triangular". Hint: a "tri-angle" has 3 sides, while a "quad-rangle" has 4 sides.