X-Dimension is the width of the narrowest element (bar or space) in a barcode. It is measured in thousands of an inch call mils - 1000 mils is 1 inch. 5 mils means 5 thousandths of an inch.
Here you can see two barcodes - on the left a 1D barcode with x-dimension of 10 mils and on the right a 2D barcode with x-dimension of 5 mils. Different scanners are able to read codes with different x-dimensions.