Your first vertex shader: waves

The vertex shader is where geometry comes alive. Add a function vert and the GPU calls it for every vertex, before any pixel is drawn:

float3 vert(float3 pos, float3 normal, float2 uv)
{
    pos.y += 0.2 * sin(pos.x * 3.0 + time);   // lift each vertex
    return pos;                               // the new position
}

pos is the vertex position in object space. Return a different one and the vertex moves. Because the plane has 180 × 180 quads, the motion looks smooth.

Lighting a displaced mesh: the mesh's stored normals still describe the flat plane, so i_normal would give wrong lighting. The fragment shader can rebuild the true surface direction from how the world position changes between neighbouring pixels: that is i_faceNormal. Use it here.