exit – I have trouble understanding this definition "form"

Does anyone explain this?

polyForm /: MakeBoxes(polyForm(e_, x_), form_) :=
With({n = Hold(e) /. p_Plus :> xForm(p, x)},
Replace(n, Hold(s_) :> MakeBoxes(s, form)))
eqn = Subtract @@
Eliminate(y == k (x - 1) && x^2/4 + y^2/3 == 1, y) == 0;
xForm /: MakeBoxes(xForm(poly_, x_), form_) :=
With({s = Series(poly, {x, Infinity, 0})},
Replace(MakeBoxes(s, form),
InterpretationBox(RowBox(a_), __) :> RowBox(a((;; -3)))))
polyForm(eqn, x)

There is no definition of "form"
I tried this below.

Replace(MakeBoxes(SeriesData(x,
DirectedInfinity(1), {3 + 4 k^2, (-8) k^2, -12 + 4 k^2}, -2, 1, 1),
form), InterpretationBox(RowBox(a_), __) :> RowBox(a((;; -3))))

Message :
MakeBoxes :: boxfmt: form in MakeBoxes ((3 + 4 k ^ 2) x ^ 2-8 k ^ 2 x + (- 12 + 4 k ^ 2) + O (1 / x) ^ 1, form) is not A type of box format. One type of box format is any member of $ BoxForms.