I don't know what this has to do with the Coder Toolbox, but anyway the dot indicates a separator between the name of the structure array "popc", and a field of that structure array called "Cost".
popc is a 2-D array (a matrix) of structures. Each structure has two or more fields (members). One of those fields is called "Cost" and another is called "Position". You can find out all of them with this command
allFieldNames = fieldnames(Cost);
That code gives a value to the Cost field of the structure located at row k, column 1 of the matrix of structures.