What is "Pass"

Code (CSharp):

1. Pass <--- what this is?
2.         {
3.             CGPROGRAM
4. #pragma vertex vert
5. #pragma fragment horizontalFrag
6. #pragma target 4.0
7.  
8.             fixed4 horizontalFrag(v2f input) : SV_Target
9.             {
10.                 return BilateralBlur(input, float2(1, 0), _QuarterResDepthBuffer, QUARTER_RES_BLUR_KERNEL_SIZE, _QuarterResTexelSize.xy);
11.             }
12.  
13.             ENDCG
14.         }

Good Morning! I would like to know how to use the Pass in the HLSL shader

 

It's mostly all explained here:
https://docs.unity3d.com/Manual/SL-Pass.html

If the pass isn't named, then the first one in the list of passes that is suitable for the build target will be used (at least on HDRP/URP, on the old pipeline it will render every unnamed pass that is compatible with the build target, one after the other). If it's named, then it can be called manually by name or is automatically used by a given render pipeline depending on the name, such as a SHADOWCASTER pass.