code interpretation on use of interp1 function for geocoding a tomogram.

I want to Geocode an image, The first part of the script is written as below, what does this part of a code S{Master}.x and S{Master}.y indicate.?

% Master position

Sy = interp1(S{Master}.x,S{Master}.y,az_profile_m);
Sz = interp1(S{Master}.x,S{Master}.z,az_profile_m);
% Terrain elevation

dem = DEM(:,a0)';
% ground range as a function of slant range

y_of_r = sqrt(rg_ax.^2 - (Sz-dem).^2) + Sy;