Insertional inactivation using pBR322
The ampicillin plate is selecting for bacteria transformed with the plasmid, either the original or recombinant, whereas the tetracycline plate is selecting only for bacteria transformed with the original plasmid. Since you are only interested in the recombinant plasmid, you need to do some form of replica plating and compare the two plates to infer which colonies contain the recombinant plasmid (those which grow on ampicillin but not on tetracycline).