c++ - I need come clarification with the HLSL with direct3d11 -

i understand in order create vertex shader , pixel shader, need create function in .fx or .hlsl file. syntax of creating such shader bit confusing.

for example, if create vertex shader this..

float4 vertexshader(float4 position : position) { return position; } 

what : position doing parameter ? assume "semantic" create filling out d3d11_input_element_desc structure?

furthermore, if create structure this

struct vs_output                         { float4 position : sv_position; }; 

what : sv_position doing member position in structure?