Generally, we distinguish between:
Variables:
x
y
And:
Parameters:
h
k
a
Parameters don't have to have any particular meaning, but the often do. Sometimes, they just suggest different members of a family of curves.
In this case:
Parameters:
h - x-coordinate of vertex of parabola
k - y-coordinate of vertex of parabola
a - Shape parameter. It is related to the distance from the vertex to the focus. There is another version of the equation of a parabola where this relationship is more clear. It appears in a different form and purpose as "2p". (or 1/a = 2p)
You should read your textbook very carefully. Parameters must be well-defined.