Question: I dont understand the position variable in shader?

[zhanglq2024]

const triangleVertices = new Float32Array([-2, -2, 2, -2, 2, 2, -2, 2]);

Why they are +-2?

position -> vPosition value never changed?

and what is meaning of the fragment shader

    float A = -dot(vPosition, vPosition);
    if (A < -4.0) discard;

Can some one give a brief explaination, appreciated.