The barcode(s) on your T1 General form contains the tax information from your T1 tax form and the other supporting forms that were used to calculate your return, in a format that CRA's computers can read.
When you print your return to mail it to CRA, not all your forms are printed. Instead, they're "condensed" in the barcodes. This allows CRA to process your return faster and with greater accuracy.
There are some circumstances where a barcode won't print on your return. If a barcode doesn't print, don't worry. Simply mail in what did print out.