Its geometric meaning is: the distance from the focus of parabola to the directrix.
In parabolic equation, p is an invariant and a fixed value.
Similarly, in elliptic equations, x and y are variables. Whoever changes first will follow suit-as long as both sides of the equation can remain equal.
A is called the (elliptical) major axis, or the semi-major axis.
B is called semi-minor axis.
These two letters are both > 0 and cannot be changed. It is called a constant.
They vividly reveal the chubby shape of the ellipse.