What is WaterTextureDepth(float3 posWS) doing in WaterCommon.hlsl?

[ZombieLei]

float WaterTextureDepth(float3 posWS)
{
    return (1 - SAMPLE_TEXTURE2D_LOD(_WaterDepthMap, sampler_WaterDepthMap_linear_clamp, posWS.xz * 0.002 + 0.5, 1).r) * (_MaxDepth + _VeraslWater_DepthCamParams.x) - _VeraslWater_DepthCamParams.x;
}

Is this function return waterPlane.y - terrain.y?
I can't get what and how this function is doing.