A is a Matrix of m rows by n columns, therefore, idx is going to be a logical vector of m rows by 1 column, if any element in the row of A is less than -M / 2 or greater than M / 2 then in that row the idx value will be 1 otherwise it will be 0.
in out those rows of A that were positive in idx are going to be saved and from those rows we want to get all the columns, and after that in those same rows that were 1 with A (idx,:) = []; what is done is to eliminate them and have a new Matrix of A without those rows.
