The actual barcode generating code is based on GNU barcode.
Just type in the string of characters to be coded, hit "Make" and the thing will pop out the barcode in PostScript. If the particular barcode supports a checksum character you may type the string with or without it. It will be calculated automatically if necessary.
I suggest you check Wikipedia for more information, as the variety of different types of barcodes might be bewildering to the novice (or indeed to anybody).