Parametric modelling is similar to 2D vector graphics formats in that instead of defining where vertices are placed in a coordinate space, it builds the model based on an instruction set that includes primitive shapes like circles but can also include complex curves defined using splines (NURBS)[0].
Constraint solving is a way of mathematically deriving the possible shapes an object can take based on the geometric constraints applied to it. For example, a 2D equilateral triangle could be defined by setting the length of one edge and then constraining all edges to be of equal length. The coordinates of the vertices are derived from these constraints.
SolveSpace [1] is an open source parametric modeler with a constraint-based solver that you can explore if this is something you're interested in.
[0] https://en.wikipedia.org/wiki/Non-uniform_rational_B-spline [1] https://solvespace.com/index.pl