You have an array dataExcel which has size(dataExcel,1) rows, and size(dataExcel,2) columns. The rows would correspond exactly to the "numbers" in excel, so row 1, 2, 3 etc. whereas the columns would correspond to the "letters" in excel, column A, B and C. (Although note they are still numbered 1, 2, 3 etc in matlab, so column A in excel would be column 1 in matlab).
So the snip of code
inflow = dataExcel(1:size(dataExcel,1)-107,4);
seeks to extract the values from row 1 to row number "size(dataExcel,1)-107", so all rows except the last 107 rows. The column index 4 then refers to... well, column 4, which would be column D in your original excel file.